lang/org: fix uninstallable centered-window-mode package

Turns out centered-window-mode isn't in any ELPA repo. Was it removed?
I'm not sure, so it's now fetched directly from its github repo.

modules/lang/org/+present.el

@@ -38,7 +38,7 @@
               :around #'+org-present*narrow-to-subtree))
 
 
-(def-package! centered-window-mode :commands centered-window-mode)
+(def-package! centered-window :commands centered-window-mode)
 
 
 ;;

modules/lang/org/packages.el

@@ -23,7 +23,7 @@
   (package! ox-pandoc))
 
 (when (featurep! +present)
-  (package! centered-window-mode)
+  (package! centered-window :recipe (:fetcher github :repo "anler/centered-window-mode"))
   (package! org-tree-slide)
   (package! ox-reveal))