doc: resolve 'magick convert' deprecation warning (#697)

Resolve the following 'magick convert' deprecation warning:

    WARNING: The convert command is deprecated in IMv7, use "magick"
    instead of "convert" or "magick convert"

Reviewed-by: NAHO <90870942+trueNAHO@users.noreply.github.com>

docs/src/tricks.md

@@ -35,7 +35,7 @@ let
   wallpaper = pkgs.runCommand "image.png" {} ''
         COLOR=$(${pkgs.yq}/bin/yq -r .base00 ${theme})
         COLOR="#"$COLOR
-        ${pkgs.imagemagick}/bin/magick convert -size 1920x1080 xc:$COLOR $out
+        ${pkgs.imagemagick}/bin/magick -size 1920x1080 xc:$COLOR $out
   '';
 in {
   stylix = {