[Docs] Update meetup contact mail address (#99321)

Arnaud is no longer active.

Author: Flakebi

llvm/docs/MeetupGuidelines.rst

@@ -18,7 +18,7 @@ General suggestions
 * We highly recommend that you join the official LLVM meetup organization. In
   addition to covering the cost of the meetup, all LLVM meetups are advertised
   together and easily found by potential attendees. Please contact
-  arnaud.degrandmaison@llvm.org for more details.
+  tanyalattner@llvm.org for more details.
 * Beware of cultural differences: what works well in one region may not work in
   other part of the world.
 * Do not be alone to organize the meetup. Try to work with a couple other