g10: Spell out --desig-revoke.

* g10/gpg.c (opts): Rename option.
* doc/gpg.texi: Update accordingly.

This is a rather long name, but I believe that this command is rarely
used, and in places where it is used frequently, the process is likely
automated.

• GnuPG-bug-id: T2700
• Signed-off-by: Justus Winter <justus@g10code.com>