Update "func" help text to GNU standards

In my earlier series to change help text to follow the GNU standards
for metasyntactic variables, I missed one: the "func" command.  This
patch updates its help text.

Tested by the buildbot.

gdb/ChangeLog
2018-06-27  Tom Tromey  <tom@tromey.com>

	* stack.c (_initialize_stack): Update "func" help text.