Fix a bug where a duplicated letter was not contributing to the
distance, if transposition was set to true and duplicated letter was the
left argument.

```python3
edit_distance("duuplicated", "duplicated", transpositions=False)
edit_distance("duplicated", "duuplicated", transpositions=True)
edit_distance("duuplicated", "duplicated", transpositions=True)
# all return 1 - correct

edit_distance("duplicated", "duuplicated", transpositions=True)
# returns 0 - incorrect
```

I believe it is a bug introduced three weeks ago by PR [2736].

The fix make nltk implementation closer to the [wikipedia] pseudo code,
which should make further reviews / iteration easier I believe.

[2736]: #2736
[wikipedia]: https://en.wikipedia.org/wiki/Damerau%E2%80%93Levenshtein_distance#Distance_with_adjacent_transpositions

Start i / j loops from 1 and not 0 to make the code closer to
[wikipedia] pseudo code, as requested by this pull request comment [0].

[wikipedia]: https://en.wikipedia.org/wiki/Damerau%E2%80%93Levenshtein_distance#Distance_with_adjacent_transpositions
[0]: #2849 (comment)