Fix documentation for benchmark

The documentation has a broken link to the pyperf package.

Author: Abhishek Ray

benchmark/README

@@ -1,7 +1,7 @@
 The batch compose and read benchmarks in this section are written using
 ``perf`` library, created by Viktor Stinner. For more information on how to get
 reliable results of test runs please consult
-http://perf.readthedocs.io/en/latest/run_benchmark.html.
+http://pyperf.readthedocs.io/en/latest/run_benchmark.html.
 
 The `simple_` benchmarks can be just run, consult command line argument on how
 to run those.