[flang] Update Google Doc link for Flang Biweekly Sync call notes

Notes from the Flang Biweekly Sync calls have been merged into the same document as the notes from the Flang Technical calls. This patch updates the link in the GettingInvolved document to point to the new location.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D124689