Fix various typos/grammer in the docs
What do yo think of the documentation btw ?
What parts would you like to see improved ? I like how concise and straightforward the docs are.

Few things that would further improve the docs IMO:
* the usage example of `Dataset.formatted_as` in https://huggingface.co/docs/datasets/master/processing.html
* the "Open in Colab" button would be nice where it makes sense (we can borrow this from the transformers project + link to HF Forum)
This PR:
* fixes various typos/grammer I came across while reading the docs
* adds the "Install with conda" installation instructions

Closes #1959 