Fix FastText doc-comment example for build_vocab and train to use correct argument names

[HLasse]

Some of the examples in the documentation for the FastText model are not runnable as they use the sentences argument which should be corpus_iterable instead.

[piskvorky]

Thanks! Is this the only place where sentences remained, could you have a look and fix them all please?

[HLasse]

Looked through fasttext.py, word2vec.py, and doc2vec.py. Could only find the references to sentences already fixed in this PR :)

[piskvorky]

Great, thanks. @mpenkov once the wheel building is fixed, can you please re-run the CI & merge? Cheers.

[mpenkov]

Merged. Thank you @HLasse !!