docstring: in python_binary(platforms=...) param doc, add an example

add an illustrative example to docstring. What I learned from https://github.com/pantsbuild/pex/issues/16

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/41992722

Bugs closed: 591

Reviewed at https://rbcommons.com/s/twitter/r/1386/
Sync of https://github.com/pantsbuild/pants/commit/cd6834f1ddc9fa80ea6dc25816b41a01c19b665a