[binutils-gdb] Update help text in linux-fork.c

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT c8a15b78e676b9101045b729a38128cbe5673701 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: c8a15b78e676b9101045b729a38128cbe5673701

Update help text in linux-fork.c

This updates some help text in linux-fork.c to conform to GNU
standards.

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

	* linux-fork.c (_initialize_linux_fork): Update help text.