Add reference to Lock

Author: serra

docs/scenarios/speed.rst

@@ -448,3 +448,4 @@ Multiprocessing
 .. _`threading`: https://docs.python.org/3/library/threading.html
 .. _`stackoverflow post`: http://stackoverflow.com/questions/26688424/python-threads-are-printing-at-the-same-time-messing-up-the-text-output
 .. _`data race`: https://en.wikipedia.org/wiki/Race_condition
+.. _`Lock`: https://docs.python.org/3/library/threading.html#lock-objects