Fix CI break from README heading-casing normalization

check:docs required the exact string '## Public contract guarantees'
but the README normalization pass retitled it to Title Case,
breaking CI. Update the manifest's required marker to match the new
canonical heading instead of reverting the casing.

Author: bradpotts

contract.manifest.json

@@ -267,7 +267,7 @@
     "requiredSectionMarkers": [
       "### Token model",
       "### Root package",
-      "## Public contract guarantees"
+      "## Public Contract Guarantees"
     ],
     "tokenContractRequiredHeadings": [
       "# Token Contract",