nixos/nix-gc, nixos/auto-upgrade: Minor documentation fixes.

The gc documentation had some artifacts (presumably being copied from
autoupgrade) and the autoupgrade docs had a typo

nixos/modules/services/misc/nix-gc.nix

@@ -39,7 +39,7 @@ in
         type = types.str;
         example = "45min";
         description = ''
-          Add a randomized delay before each automatic upgrade.
+          Add a randomized delay before each garbage collection.
           The delay will be chosen between zero and this value.
           This value must be a time span in the format specified by
           <citerefentry><refentrytitle>systemd.time</refentrytitle>

nixos/modules/tasks/auto-upgrade.nix

@@ -90,7 +90,7 @@ in {
         example = "45min";
         description = ''
           Add a randomized delay before each automatic upgrade.
-          The delay will be chozen between zero and this value.
+          The delay will be chosen between zero and this value.
           This value must be a time span in the format specified by
           <citerefentry><refentrytitle>systemd.time</refentrytitle>
           <manvolnum>7</manvolnum></citerefentry>