chore(bors.toml): only cut PR bodies from commit messages if --- starts a line (leanprover-community#34684)

Currently bors cuts the rest of the PR body from the commit message if it contains `---` anywhere in the message, including in formatting markdown tables (cf. leanprover-community#33456 and commit fa0ba71). Adding `\n` here should prevent bors from mangling such messages.

Author: bryangingechen

bors.toml

@@ -4,5 +4,5 @@ timeout_sec = 7200
 block_labels = ["WIP", "blocked-by-other-PR", "merge-conflict", "awaiting-CI"]
 delete_merged_branches = true
 update_base_for_deletes = true
-cut_body_after = "---"
+cut_body_after = "\n---"
 max_batch_size = 16