Docs: Improve example for ``itertools.batched()`` (#136775)

The current example `batched('ABCDEFG', n=3) → ABC DEF G` can confuse readers because both, the size of the tuples and the number of tuples are 3.
By using a batch size of n=2, it is clearer that the `n` argument refers to the size of the resulting tuples.
I.e. the new example is: `batched('ABCDEFG', n=2) → AB CD EF G`