Remove angle brackets from "apropos" text

I didn't notice during review, but the boxing patch introduced this
change:

-	      styled_string (command_style.style (), "apropos word"));
+	      styled_string (command_style.style (), "apropos <word>"));

GNU doesn't generally use the "<...>" convention, but instead uses
upper case for "metasyntactic variables".  See the GNU coding
standards for this topic.

In this particular spot, though, I think the old form using quotes is
preferable.  This patch reverts this change.

Reviewed-By: Tom de Vries <tdevries@suse.de>