pyenv-virtualenv: add activate/deactivate examples (#3842)

pages/common/pyenv-virtualenv.md

@@ -5,8 +5,16 @@
 
 - Create a new Python 3.6.6 virtual environment:
 
-`pyenv virtualenv {{3.6.6}} {{my-virtual-env-3.6.6}}`
+`pyenv virtualenv {{3.6.6}} {{virtualenv_name}}`
 
 - List all existing virtual environments:
 
 `pyenv virtualenvs`
+
+- Activate a virtual environment:
+
+`pyenv activate {{virtualenv_name}}`
+
+- Deactivate the virtual environment:
+
+`pyenv deactivate`