[binutils-gdb] Update help text in dcache.c

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT ffb2b66cabe9deefefa1312ef0b7bb064656e611 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: ffb2b66cabe9deefefa1312ef0b7bb064656e611

Update help text in dcache.c

This updates some text in dcache.c to follow GNU standards.
Here, I found a couple of spots needing an update.

2018-04-29  Tom Tromey  <tom@tromey.com>

	* dcache.c (dcache_info_1): Update usage text.
	(_initialize_dcache): Update help text.