Fix funding

.github/FUNDING.yml

@@ -1,2 +1,2 @@
 patreon: assimp
-Paypal: https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=4JRJVPXC4QJM4
+custom: https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=4JRJVPXC4QJM4