Fix to use "." to source script files

Adhering to coding conventions. Refer to ``Code conventions`` at
https://docs.openstack.org/contributor-guide/ for details.

Change-Id: I23ff70c9caefc870b3cc9d61cd8c18b534f2ffaf

doc/source/contributor/plugin/base-setup.rst

@@ -34,7 +34,7 @@ First off, we need to create the project structure. To do so, we can use
 generate the skeleton of our project::
 
     $ virtualenv thirdparty
-    $ source thirdparty/bin/activate
+    $ . thirdparty/bin/activate
     $ pip install cookiecutter
     $ cookiecutter https://github.com/openstack-dev/cookiecutter