gdk-pixbuf: update caveats to clarify how to manually update cache

Closes Homebrew/legacy-homebrew#33166.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/gdk-pixbuf.rb

@@ -53,7 +53,7 @@ def post_install
   def caveats; <<-EOS.undent
     Programs that require this module need to set the environment variable
       export GDK_PIXBUF_MODULEDIR="#{HOMEBREW_PREFIX}/lib/gdk-pixbuf-2.0/2.10.0/loaders"
-    If you need to manually update the query loader cache
+    If you need to manually update the query loader cache, set GDK_PIXBUF_MODULEDIR then run
       #{bin}/gdk-pixbuf-query-loaders --update-cache
     EOS
   end