Fix documentation for benchmark
The documentation has a broken link to the pyperf package.
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.
