rest-api-changes.txt: Fix case of mark-private anchor

Change-Id: I795225fe7fba7bfdd4dbf944a65aacfa1b520eab
diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index fdcae26..5d32eba 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -2107,7 +2107,7 @@
   }
 ----
 
-[[Mark-private]]
+[[mark-private]]
 === Mark Private
 --
 'PUT /changes/link:#change-id[\{change-id\}]/private'