Mark len call as a code snippet in stdtypes.rst. (GH-9804)

Doc/library/stdtypes.rst

@@ -3809,7 +3809,7 @@ copying.
 
       ``nbytes == product(shape) * itemsize == len(m.tobytes())``. This is
       the amount of space in bytes that the array would use in a contiguous
-      representation. It is not necessarily equal to len(m)::
+      representation. It is not necessarily equal to ``len(m)``::
 
          >>> import array
          >>> a = array.array('i', [1,2,3,4,5])