Fix bug in documentation: use pip3 instead of pip

README.rst

@@ -34,7 +34,7 @@ Setup a virtualenv for development and install it in editable mode::
 
     # install in development environment
     virtualenv --python=python3 venv; source venv/bin/activate;
-    pip install -e .
+    pip3 install -e .
 
     # run tests
     python3 setup.py test