-F flage of plot: Update figure caption corresponding to GMT 6.5

[yvonnefroehlich]

Description of proposed changes

Over time, the general syntax of the -F flage of plot has changed:

GMT 6.2 -F[c|n|r][a|f|s|r|refpoint]
GMT 6.3 -F[c|n|p][a|f|s|r|refpoint]
GMT 6.5 -F[c|n|p][a|r|s|t|refpoint]

I am wondering whether the last line of the figure caption in the GMT documentation has to be updated from
"directives ra, rt, rs, r10/35 and na" to
"directives pa, pt, ps, p10/35 and na"?

Reminders

• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.

[PaulWessel]

Yes, comments in gmt_init.c say:

case 'p': case 'r': k = 1; S->method = SEGM_REFPOINT; break; /* Backwards support for r for refpoint, now p for point */