Remove literal from description, which was failing the action (#3789)

Signed-off-by: Christian Petersen <github@fonz.me>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

.github/actions/sign-image/action.yaml

@@ -18,9 +18,9 @@ inputs:
         required: true
     run-started-at:
         description: >-
-          ISO-8601 workflow run start time, typically `${{ github.run_started_at }}`
-          from the caller. Recorded as `runDetails.metadata.startedOn` in the
-          SLSA provenance predicate.
+          ISO-8601 workflow run start time (typically
+          `github.run_started_at` from the caller workflow). Recorded as
+          `runDetails.metadata.startedOn` in the SLSA provenance predicate.
         required: true
 
 runs: