Fix link to virtualenv install page in README.rst
Fix link to virtualenv install page in README.rst

Winterflower/khmer,ged-lab/khmer,souravsingh/khmer,souravsingh/khmer,F1000Research/khmer,F1000Research/khmer,ged-lab/khmer,ged-lab/khmer,souravsingh/khmer,Winterflower/khmer,Winterflower/khmer,F1000Research/khmer