Fix warning formatting

Missing a space and a `f` here

Reviewers: jerry,brian-k
Topic: revup-f

revup/topic_stack.py

@@ -746,8 +746,8 @@ async def mark_rebases(self, skip_rebase: bool) -> None:
                 and review.remote_base != review.pr_info.baseRef
             ):
                 logging.warning(
-                    f"Branch {review.remote_head} was merged into {review.pr_info.baseRef}"
-                    "instead of {review.remote_base} as expected!"
+                    f"Branch {review.remote_head} was merged into {review.pr_info.baseRef} "
+                    f"instead of {review.remote_base} as expected!"
                 )
                 # NOTE: This may not iteract well with the check at the end of create_commits
                 # but they are both corner cases and the worst that could happen is we fail to