[BE] Fix rule not found error message (#101745)

Prevent error message from becoming of single column of characters

Thanks @clee200 for explaining how it worked before

<!--
copilot:poem
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### <samp>🤖 Generated by Copilot at fef1e25</samp>

> _`reject_reason` fixed_
> _Syntax error caused trouble_
> _Autumn of bugs ends_

Pull Request resolved: https://github.com/pytorch/pytorch/pull/101745
Approved by: https://github.com/kit1980, https://github.com/osalpekar
diff --git a/.github/scripts/trymerge.py b/.github/scripts/trymerge.py
index f8670a6..6e17b3b 100755
--- a/.github/scripts/trymerge.py
+++ b/.github/scripts/trymerge.py
@@ -1262,9 +1262,9 @@
                 reject_reason_score = num_matching_files
                 reject_reason = "\n".join(
                     (
-                        f"Not all files match rule `{rule_name}`."
-                        f"{num_matching_files} files matched, but there are still non-matching files:"
-                        f"{','.join(non_matching_files[:5])}{', ...' if len(non_matching_files) > 5 else ''}"
+                        f"Not all files match rule `{rule_name}`.",
+                        f"{num_matching_files} files matched, but there are still non-matching files:",
+                        f"{','.join(non_matching_files[:5])}{', ...' if len(non_matching_files) > 5 else ''}",
                     )
                 )
             continue