Update facebook.mdx

Correct url

docs/sources/companion-plugins/facebook.mdx

@@ -84,7 +84,7 @@ new Uppy()
 ### Use in Companion
 
 You can create a Facebook App on the
-[Facebook Developers site](https://developers.facebook.com/).
+[Facebook Developers site](https://developers.facebook.com/apps).
 
 The app page has a “Redirect URIs” field. Here, add: