fixed spelling mistake from 'uisng' to 'using'. (#6168)

Author: Jongkeun

py/docs/source/index.rst

@@ -100,7 +100,7 @@ Example 1:
 Example 2:
 ==========
 
-Selenium WebDriver is often used as a basis for testing web applications.  Here is a simple example uisng Python's standard `unittest <http://docs.python.org/3/library/unittest.html>`_ library:
+Selenium WebDriver is often used as a basis for testing web applications.  Here is a simple example using Python's standard `unittest <http://docs.python.org/3/library/unittest.html>`_ library:
 
 .. code-block:: python