[gdb] Fix set/show style metadata help text

There's a recent regression:
...
FAIL: gdb.gdb/unittest.exp: maintenance selftest
...

In more detail:
...
Running selftest help_doc_invariants.^M
help doc broken invariant: command 'set style metadata' help doc first line \
  is not terminated with a '.' character^M
help doc broken invariant: command 'show style metadata' help doc first line \
  is not terminated with a '.' character^M
Self test failed: self-test failed at gdb/unittests/help-doc-selftests.c:95^M
...

Fix this by adding a '.' at the end of the first line of the help text for
set/show style metadata.

Tested on x86_64-linux.

gdb/ChangeLog:

2019-10-03  Tom de Vries  <tdevries@suse.de>

	* cli/cli-style.c (_initialize_cli_style): Adding a '.' at the end of
	the first line of the help text for set/show style metadata.