[gdb/testsuite] Fix layout next/prev/regs help message

[gdb/testsuite] Fix layout next/prev/regs help message

With test-case gdb.gdb/unittest.exp, I run into:
...
(gdb) maintenance selftest^M
   ...
Running selftest help_doc_invariants.^M
help doc broken invariant: command 'layout next' help doc first line is \
  not terminated with a '.' character^M
help doc broken invariant: command 'layout prev' help doc first line is \
  not terminated with a '.' character^M
help doc broken invariant: command 'layout regs' help doc first line is \
  not terminated with a '.' character^M
Self test failed: self-test failed at help-doc-selftests.c:95^M
...

Fix this by adding the missing '.' character.

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-02-24  Tom de Vries  <tdevries@suse.de>

	* tui/tui-layout.c (_initialize_tui_layout): Fix help messages for
	commands layout next/prev/regs.