Fix typo in python docs

py/docs/source/index.rst

@@ -40,7 +40,7 @@ Alternately, you can download the source distribution from `PyPI <http://pypi.py
 
     python setup.py install
 
-Note: both of the methods described above install `selenium` as a system-wide package  That will require administrative/root access to there machine.  You may consider using a `virtualenv <http://www.virtualenv.org/>`_ to create isolated Python environments instead.
+Note: both of the methods described above install `selenium` as a system-wide package  That will require administrative/root access to their machine.  You may consider using a `virtualenv <http://www.virtualenv.org/>`_ to create isolated Python environments instead.
 
 Example 0:
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