Fix typo in multiprocessing documentation, thanks to Dmitry Tolstoy from docs@

Doc/library/multiprocessing.rst

@@ -279,7 +279,7 @@ For example::
        return x*x
 
    if __name__ == '__main__':
-       with Pool(processes=4) as pool         # start 4 worker processes
+       with Pool(processes=4) as pool:        # start 4 worker processes
            result = pool.apply_async(f, [10]) # evaluate "f(10)" asynchronously
            print(result.get(timeout=1))       # prints "100" unless your computer is *very* slow
            print(pool.map(f, range(10)))      # prints "[0, 1, 4,..., 81]"