Fix a typo in code example

Author: ludovic-gasc

README.rst

@@ -18,7 +18,7 @@ provides useful fixtures and markers to make testing easier.
 .. code-block:: python
 
     @pytest.mark.asyncio
-    def test_some_asyncio_code()
+    def test_some_asyncio_code():
         res = yield from library.do_something()
         assert b'expected result' == res