[gdb/selftest] Fix 'help_doc_invariants' self-test

  Running selftest help_doc_invariants.
  help doc broken invariant: command 'signal-event' help doc has over-long line
  Self test failed: self-test failed at unittests/command-def-selftests.c:121

  The reason is that doc string of 'signal-event' doesn't have
  newlines at end of its line.  Fix by adding newlines.