Prefix Pip commands with python -m in docs

This ensures Pip commands will be executed for the current Python
interpreter and not, say, whichever Python interpreter happens to be
associated with `/usr/local/bin/pip`.

docs/contribute.rst

@@ -44,9 +44,9 @@ to manually create and activate a virtual environment::
 
 Install the needed dependencies and set up the project::
 
-    pip install invoke
+    python -m pip install invoke
     invoke setup
-    pip install -e ~/projects/pelican
+    python -m pip install -e ~/projects/pelican
 
 Your local environment should now be ready to go!