[binutils-gdb] Update help strings in skip.c

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 784e4b3e80c1093203ae1a0da33be7a670942713 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: 784e4b3e80c1093203ae1a0da33be7a670942713

Update help strings in skip.c

This changes help strings in skip.c to follow the GNU style.

2018-04-27  Tom Tromey  <tom@tromey.com>

	* skip.c (_initialize_step_skip): Update help strings.