Fix leftover comment

Author: ldionne

libcxx/test/benchmarks/containers/flat_map.bench.cpp

@@ -16,7 +16,7 @@
 
 int main(int argc, char** argv) {
   support::associative_container_benchmarks<std::flat_map<int, int>>("std::flat_map<int, int>");
-  // support::associative_container_benchmarks<std::map<std::string, int>>("std::map<std::string, int>");
+  support::associative_container_benchmarks<std::flat_map<std::string, int>>("std::flat_map<std::string, int>");
 
   benchmark::Initialize(&argc, argv);
   benchmark::RunSpecifiedBenchmarks();