Minor documentation revisions from copyediting
# What does this PR do?

Minor changes to the documentation to correct typos and improve readability. I noticed these typos while reading through the docs to familiarize myself with the library for a project, and thought it would be nice to make a PR for them 😊

I've already tested building the docs from these changes, and all changes seem to have taken effect properly 👍 

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests), Pull Request section?

## Who can review?

@sgugger would you mind reviewing this PR? I'm happy to make any changes (or remove any changes) you want 🙂
Looks like you need to run `make style` on your branch to fix the formatting of the doc files. Let me know if you run into any trouble doing that.<|||||>Thanks @sgugger 😄 I was able to run `make style` successfully (and update `preprocessing.rst` with the changes), but it looks like the `check_code_quality` check ran out of memory this time 😅 

![Screen Shot 2020-12-23 at 9 33 15 AM](https://user-images.githubusercontent.com/1848731/103007806-0e791700-4502-11eb-9f2c-b1946dc64dbf.png)

I tried rerunning it, but it seems like I don't have permission. Could you try rerunning it?

I'm also happy to bump the `resource_class` for the check from `medium` to `medium+` if that would be helpful 🙂 <|||||>Yes it was just a spurious failure. Thanks!