Bug #1685704: use -m switch in timeit docs.

Doc/lib/libtimeit.tex

@@ -127,7 +127,7 @@ function and run its \method{timeit} method with \var{number} executions.
 When called as a program from the command line, the following form is used:
 
 \begin{verbatim}
-python timeit.py [-n N] [-r N] [-s S] [-t] [-c] [-h] [statement ...]
+python -m timeit [-n N] [-r N] [-s S] [-t] [-c] [-h] [statement ...]
 \end{verbatim}
 
 where the following options are understood: