Fixing typo in class doc
Fixing typo in class doc

Edits suggested related to: https://github.com/google/seq2seq/blob/master/seq2seq/decoders/attention.py#L78 . 

There is probably other issues in this doc i.e. there is no `cell`, `helper` nor `initial_state` defined in `__init__()`.
google/seq2seq,chunfengh/seq2seq,chunfengh/seq2seq,liyi193328/seq2seq,liyi193328/seq2seq,shashankrajput/seq2seq,shashankrajput/seq2seq,google/seq2seq,liyi193328/seq2seq,shashankrajput/seq2seq,google/seq2seq,shashankrajput/seq2seq,chunfengh/seq2seq,liyi193328/seq2seq,google/seq2seq,chunfengh/seq2seq,liyi193328/seq2seq