PULL_REQUEST_TEMPLATE.md: tell the user to remove the welcome comment (#2811)

We're now validating in
#2810 that it is
removed, so we should tell the person to remove it


## Release notes

Co-authored-by: Andreas Arvidsson <[email protected]>

Author: phillco

.github/PULL_REQUEST_TEMPLATE.md

@@ -9,8 +9,9 @@ Thanks for your change!
 
 * Remember to *test the cheatsheet manually* if you've added a new action,
   modifier, or scope -- or changed any Talon files.
--->
 
+Remove this comment.
+-->
 
 
 ## Release notes