Repoint marl branch from master to main

See https://github.com/google/marl/issues/153 for more details

Bug: b/140546382
Change-Id: I73cd3e575b781890e82eda94285e8a585bd879d5
diff --git a/third_party/update-marl.sh b/third_party/update-marl.sh
index 41dc656..4ba5518 100755
--- a/third_party/update-marl.sh
+++ b/third_party/update-marl.sh
@@ -9,7 +9,7 @@
   REASON="\n$REASON\n"
 fi
 
-git subtree pull --prefix third_party/marl https://github.com/google/marl master --squash -m "Update marl"
+git subtree pull --prefix third_party/marl https://github.com/google/marl main --squash -m "Update marl"
 
 ALL_CHANGES=`git log -n 1 HEAD^2 | egrep '^(\s{4}[0-9a-f]{9}\s*.*)$'`
 HEAD_CHANGE=`echo "$ALL_CHANGES" | egrep '[0-9a-f]{9}' -o -m 1`