Add review permissions for rustfmt team members

Expand the permissions to apply to all rustfmt team members, not just
the team lead. These review permissions will only be used for rustfmt
subtree syncs and the occasional in-tree rustfmt PR.

Author: ytmimi

teams/rustfmt.toml

@@ -16,7 +16,7 @@ alumni = [
 [[github]]
 orgs = ["rust-lang"]
 
-[leads-permissions]
+[permissions]
 # Primarily for subtree syncs and other rustfmt-related review.
 bors.rust.review = true