gdb/doc: down case contents of @var

gdb/doc: down case contents of @var

After a discussion on a recent patch it was pointed out that the
contents of a @var should (generally) be lower case.  I took a look
through the GDB manual and there are a small number of places where
the contents are currently upper case, but one in particular seemed
like an obvious candidate for being down cased, so lets do that.

gdb/doc/ChangeLog:

	* gdb.texinfo (PowerPC Embedded): Down case contents of @var.