Bug 1681823 - Fix typo in clang-tidy-external Treeherder symbol r=andi DONTBUILD

I noticed this while trying to "add new jobs" for something nearby.

Differential Revision: https://phabricator.services.mozilla.com/D99385

taskcluster/ci/toolchain/clang-tidy.yml

@@ -103,7 +103,7 @@ linux64-clang-tidy-external:
     index:
         job-name: linux64-clang-tidy-external
     treeherder:
-        symbol: TL(linux66-clang-tidy-e)
+        symbol: TL(clang-tidy-e)
     worker:
         artifacts:
             - name: project/civet/clang-tidy.tar.zst