Subject: doc: Incorrect link to gpm manual

From: Leo Famulari <[email-masked]>
Date: Sun, 7 Feb 2021 15:23:30 -0500

In our manual section Base Services, the link to gpm's manual is
incorrect.

It points to
<https://guix.gnu.org/manual/en/gpm/Command-Line.html#Command-Line>, but
that returns 404. 

Presumably it should point to some server besides our own.