Add tidelift funding for icalendar

This adds funding for icalendar on tidelift

Fixes #771 
See also #754

.github/FUNDING.yml

@@ -4,7 +4,7 @@ github: # Replace with up to 4 GitHub Sponsors-enabled usernames e.g., [user1, u
 patreon: # Replace with a single Patreon username
 open_collective: python-icalendar
 ko_fi: # Replace with a single Ko-fi username
-tidelift: # Replace with a single Tidelift platform-name/package-name e.g., npm/babel
+tidelift: pypi/icalendar # Replace with a single Tidelift platform-name/package-name e.g., npm/babel
 community_bridge: # Replace with a single Community Bridge project-name e.g., cloud-foundry
 liberapay: # Replace with a single Liberapay username
 issuehunt: # Replace with a single IssueHunt username