[virtualenv] Fix pipenv's shell prompt

When we launch pipenv's shell prompt, pipenv automatically adds the
virtual environment name in the shell prompt, like virtualenv.

Author: etanot

docs/virtualenv.rst

@@ -169,7 +169,7 @@ the following Python code in a file named *main.py*.
 ::
 
     $ pipenv shell
-    $ $ python main.py
+    (myproject-dbBcpQ4l) $ python main.py
     Your IP is 192.168.1.2
 
 Exiting from the virtualenv