Fixup freeze recipe for Pex 2.x.
Fixes
@@ -68,7 +68,7 @@ Or instead freeze your current virtualenv via requirements.txt and execute it an
.. code-block:: bash
- $ pex -r <(pip freeze) -o my_virtualenv.pex
+ $ pex $(pip freeze) -o my_virtualenv.pex
$ deactivate
$ ./my_virtualenv.pex
