fix remote upstream link

Author: Phe0

developer_docs/preparing_a_pull_request.md

@@ -19,7 +19,7 @@ Make sure you're tracking upstream p5.js repository.
 
 If you see an error, you'll need to start tracking the main p5.js repo as an "upstream" remote repository. You'll only need to do this once! But, no harm is done if you run it a second time.
 
-    git remote add upstream https://github.com/processing/p5.js
+    git remote add upstream https://github.com/processing/p5.js-web-editor
 
 Then ask git about the latest changes.