[API] correct link for flush_synced docs

rest-api-spec/api/indices.flush_synced.json

@@ -1,6 +1,6 @@
 {
   "indices.flush_synced": {
-    "documentation": "http://www.elastic.co/guide/en/elasticsearch/reference/master/indices-flush.html",
+    "documentation": "http://www.elastic.co/guide/en/elasticsearch/reference/master/indices-synced-flush.html",
     "methods": ["POST", "GET"],
     "url": {
       "path": "/_flush/synced",