Fix unittest.exp failure due to 'set debuginfod' addition

The 'set debuginfod' change caused a regression in unittest.exp:

    Running selftest help_doc_invariants.
    help doc broken invariant: command 'info set debuginfod' help doc first line is not terminated with a '.' character
    help doc broken invariant: command 'set debuginfod' help doc first line is not terminated with a '.' character
    help doc broken invariant: command 'show debuginfod' help doc first line is not terminated with a '.' character
    Self test failed: self-test failed at ../../binutils-gdb/gdb/unittests/command-def-selftests.c:100

This patch fixes the problem.  I'm checking it in.

Author: tromey

gdb/debuginfod-support.c

@@ -406,8 +406,8 @@ _initialize_debuginfod ()
 {
   /* set/show debuginfod */
   add_setshow_prefix_cmd ("debuginfod", class_run,
-			  _("Set debuginfod options"),
-			  _("Show debuginfod options"),
+			  _("Set debuginfod options."),
+			  _("Show debuginfod options."),
 			  &set_debuginfod_prefix_list,
 			  &show_debuginfod_prefix_list,
 			  &setlist, &showlist);