README: remove incorrect hookwrapper

A hookwrapper requires a `yield`, but seems unnecessary here, so just remove it.

Author: bluetech

README.rst

@@ -127,6 +127,6 @@ Development
 
   .. code-block:: python
 
-    @hookimpl(tryfirst=True, hookwrapper=True)
+    @hookimpl(tryfirst=True)
     def pytest_runtest_makereport(item, call):
         print(item.execution_count)