Fix code

Author: Simran-B

3.12/aql/execution-and-performance-optimizer.md

@@ -506,7 +506,7 @@ applied multiple times at different optimization stages.
       var url = "/_api/query/rules";
       var rules = internal.arango.GET(url);
       assert(Array.isArray(rules));
-      assert(rules.some(e => e.clusterOnly));
+      assert(rules.some(e => e.flags && e.flags.clusterOnly));
       var outfile = "Documentation/optimizer-rules.json"
       assert(fs.write(outfile, JSON.stringify(rules, undefined, 2)));
     @END_EXAMPLE_ARANGOSH_RUN