Use try-finally idiom in example for locks in multiprocessing

Doc/library/multiprocessing.rst

@@ -262,7 +262,9 @@ that only one process prints to standard output at a time::
 
    def f(l, i):
        l.acquire()
+       try:
            print('hello world', i)
+       finally:
            l.release()
 
    if __name__ == '__main__':