Update GitHub sponsor link to mempool.space/sponsor

.github/FUNDING.yml

@@ -9,4 +9,4 @@ community_bridge: # Replace with a single Community Bridge project-name e.g., cl
 liberapay: # Replace with a single Liberapay username
 issuehunt: # Replace with a single IssueHunt username
 otechie: # Replace with a single Otechie username
-custom: ['https://mempool.space/about'] # Replace with up to 4 custom sponsorship URLs e.g., ['link1', 'link2']
+custom: ['https://mempool.space/sponsor'] # Replace with up to 4 custom sponsorship URLs e.g., ['link1', 'link2']