Update contributing.md (#1023)
Update contributing.md (#1023)

"If you decided to follow the suggestion about about the Virtualenv *and* you've run `source bin/activate` ..." --> "If you decided to follow the suggestion about the Virtualenv *and* you've run `source bin/activate` within your new virtualenv directory to activate..."
plotly/python-api,plotly/plotly.py,plotly/python-api,plotly/plotly.py,plotly/plotly.py,plotly/python-api