Fix links on README.md

README.md

 pprof can read `perf.data` files generated by the
 [Linux perf](https://perf.wiki.kernel.org/index.php) tool by using the
 `perf_to_profile` program from the
-(perf_data_converter)[http://github.com/google/perf_data_converter] package.
+[perf_data_converter](http://github.com/google/perf_data_converter) package.
 
 ## Further documentation