twelf-mode: s/string-to-int/string-to-number/

[cdrichards]

Makes twelf-next-error and friends work on Emacs >= 26.1, which removed string-to-int.

The string-to-int function was an obsolete alias for string-to-number, so this change is backwards-compatible.

Tested manually with Emacs 30.2:

• Before: twelf-next-error fails with symbol’s function definition is void: string-to-int
• After: twelf-next-error goes to the next error.

[robsimmons]

Thanks Christina!