DOC: fix cython example (failed on Windows) (GH6002)
Closes  #6002

The signature of `apply_integrate_f_wrap` (http://pandas-docs.github.io/pandas-docs-travis/enhancingperf.html#more-advanced-techniques) used `Py_ssize_t` but on Windows that gives an error, as the dataframe is generated with numpy random arrays, which gives int32 on Windows, while `Py_ssize_t` is int64 on 64 bit Windows.
