mattermost: cosmetic: remove stray character (#17897)

nixos/modules/services/web-apps/mattermost.nix

···
         type = types.str;
         example = "https://chat.example.com";
         description = ''
-          URL this Mattermost instance is reachable under, without trailing slash."
+          URL this Mattermost instance is reachable under, without trailing slash.
         '';
       };