update links to 6.2

@peastman: I pushed the 6.2.0 docs already. This updates the links

- http://docs.openmm.org/6.2.0/userguide/index.html
- http://docs.openmm.org/6.2.0/developerguide/index.html
- http://docs.openmm.org/6.2.0/api-python/annotated.html
- http://docs.openmm.org/6.2.0/api-c++/namespaceOpenMM.html