Figure 3: Update with new Figure.subplot syntax

[seisman]

Update after GenericMappingTools/pygmt#4313 is merged.

[gitnotebooks]

Found 1 changed notebook. Review the changes at https://app.gitnotebooks.com/GenericMappingTools/pygmt-paper-figures/pull/32

[seisman]

I've updated autolabel="(a)+o0.15c/0.3c+gwhite@30+v" to

    tag="(a)",
    tag_position=Position("TL", offset=(0.15, 0.3)),
    tag_orientation="vertical",

+gwhite@30 sets the tag box and is omitted here to prevent the code from becoming too long.

The new figure looks like below: