gdb: fix error message for $_gdb_maint_setting

gdb: fix error message for $_gdb_maint_setting

I spotted this behaviour:

  (gdb) p $_gdb_maint_setting("xxx")
  First argument of $_gdb_maint_setting must be a valid setting of the 'show' command.

Notice that GDB claims I need to use a setting from the 'show'
command, which isn't correct for $_gdb_maint_setting, in this case I
need to use a setting from 'maintenance show'.

This same issue is present for $_gdb_maint_setting_str.

This commit fixes this minor issue.

Approved-By: Simon Marchi <simon.marchi@efficios.com>