Update GMT_App_N_1.sh avoiding all-uppercase characters in illustration

[CovMat]

The use of all-uppercase characters in illustration may cause user confusion.

~/Desktop/GMT/test 🅒 gmtdocs                                                                                                          
❯ echo 0 0 | gmt plot -SkCITY/1c -R-1/1/-1/1 -Gblack -W1p -png tmp
plot [ERROR]: Could not find either custom symbol or EPS macro CITY
plot [ERROR]: Option -S: Parsing failure

~/Desktop/GMT/test 🅒 gmtdocs                                                                                                          
❯ echo 0 0 | gmt plot -Skcity/1c -R-1/1/-1/1 -Gblack -W1p -png tmp

~/Desktop/GMT/test 🅒 gmtdocs                                                                                                          
❯ cd ~/miniforge3/envs/gmtdocs/share/gmt/custom/

~/miniforge3/envs/gmtdocs/share/gmt/custom 🅒 gmtdocs                                                                                  
❯ cat gmt_custom_symbols.conf
#
#       List of GMT Custom Symbols that are distributed with
#       GMT and kept in the share/custom directory.  To add more
#       symbols, create the *.def file, place it in share/custom,
#       and add a 1-line entry (alphabetically) in this list.
#----------------------------------------------------------------
astroid        : 4-pointed concave star symbol
circle         : Circle (similar to -Sc)
city           : City symbol (circle in circle)
cross          : Cross (similar to -Sx)
crosshair      : Bullseye crosshair
deltoid        : 3-pointed concave triangle symbol
diamond        : Diamond (similar to -Sd)
flash          : Lightning flash symbol
hexagon        : Hexagon (similar to -Sh)
hlens          : Horizontal convex lens symbol
hlozenge       : Narrow horizontal diamond-like symbol
hneedle        : Horizontal, very narrow diamond
hurricane      : Hurricane symbol
lcrescent      : Crescent with belly to the left
lflag          : Post with flag to the left
ltriangle      : Triangle pointing to the left
meca           : Focal mechanism (beachball)
octagon        : Octagon (similar to -Sg)
pacman         : A Pacman symbol
pentagon       : Pentagon (similar to -Sn)
rcrescent      : Crescent with belly to the right
rflag          : Post with flag to the right
rtriangle      : Triangle pointing to the right
sectoid        : Concave, triangular sector
square         : Square (similar to -Ss)
squaroid       : Concave square
star           : Star (similar to -Sa)
star3          : 3-pointed triangular star symbol
star4          : 4-pointed diagonal star symbol
starp          : 4-pointed polar star
sun            : Shining sun symbol
triangle       : Triangle (similar to -St)
trirot1        : 90-degree triangle in 1st quadrant
trirot2        : 90-degree triangle in 2nd quadrant
trirot3        : 90-degree triangle in 3rd quadrant
trirot4        : 90-degree triangle in 4th quadrant
vlens          : Vertical convex lens symbol
vlozenge       : Narrow vertical diamond-like symbol
vneedle        : Vertical, very narrow diamond
volcano        : Active with some bad fume bubbles
QR             : GMT QR code (opaque)
QR_transparent : GMT QR code (transparent)

[CovMat]

This script generates image at https://docs.generic-mapping-tools.org/latest/reference/custom-symbols.html#custom-symbols

[joa-quim]

I think you are right.

Any more opinions?