rnn and backprop fix

Merge pull request d2l-ai#213 from astonzhang/nin

consistent indentation

Update index.md

fix latex bug

update data_iter (d2l-ai#87)

Fix typo (d2l-ai#66)

Update index.md

fix small typo in docs (d2l-ai#19)

* fix typo in why.md

* fix typo in introduction.md

* fix typo in ndarray.md