[binutils-gdb] Update help text in record-btrace.c

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 55063ddb8e9a3179b65047db21f4c8607782d075 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: 55063ddb8e9a3179b65047db21f4c8607782d075

Update help text in record-btrace.c

This updates the help text in record-btrace.c to conform to GNU
standards.

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

	* record-btrace.c (_initialize_record_btrace): Update help text.