docs: Fix typo in README.

Signed-off-by: Michael Seifert <m.seifert@digitalernachschub.de>

README.rst

@@ -251,7 +251,7 @@ Note about unittest
 -------------------
 
 Test classes subclassing the standard `unittest <https://docs.python.org/3/library/unittest.html>`__ library are not supported, users
-are recommended to use `unitest.IsolatedAsyncioTestCase <https://docs.python.org/3/library/unittest.html#unittest.IsolatedAsyncioTestCase>`__
+are recommended to use `unittest.IsolatedAsyncioTestCase <https://docs.python.org/3/library/unittest.html#unittest.IsolatedAsyncioTestCase>`__
 or an async framework such as `asynctest <https://asynctest.readthedocs.io/en/latest>`__.
 
 Contributing