Use upper case for metasyntactic variables in "help find"

Use upper case for metasyntactic variables in "help find"

While answering a user's question on irc, I realized that the
metasyntactic variables in "help find" are not in upper case.  As you
know this is one of my pet quests, so here is a patch to fix this.
Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-02-20  Tom Tromey  <tromey@adacore.com>

	* findcmd.c (_initialize_mem_search): Use upper case for
	metasyntactic variables.