Update docs: hprof script no longer uses Python 2
Update docs: hprof script no longer uses Python 2

Summary:
GitHub issue [#567](https://github.com/facebook/redex/issues/567)

Our OSS documentation still asks the user to use Python 2 to run our python script, which has been python 3 internally. Fixing the documentation.

Reviewed By: agampe

Differential Revision: D24804743

fbshipit-source-id: c515c0f4ff7231a9a6a901b1d7cc7270f22683d6

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex