slightly extend PR creation message

Author: RalfJung

src/tools/miri/triagebot.toml

@@ -20,7 +20,7 @@ contributing_url = "https://github.com/rust-lang/miri/blob/master/CONTRIBUTING.m
 [assign.custom_welcome_messages]
 welcome-message = "(unused)"
 welcome-message-no-reviewer = """
-Thank you for contributing to Miri!
+Thank you for contributing to Miri! A reviewer will take a look at your PR, typically within a week or two.
 Please remember to not force-push to the PR branch except when you need to rebase due to a conflict or when the reviewer asks you for it.
 """