Docs typos fixes

[tienpham94]

Thanks for contributing to react-virtualized!

Before submitting a pull request, please complete the following checklist:

• The existing test suites (npm test) all pass
• For any new features or bug fixes, both positive and negative test cases have been added
• For any new features, documentation has been added
• For any documentation changes, the text has been proofread and is clear to both experienced users and beginners.
• Format your code with prettier (npm run prettier).
• Run the Flow typechecks (npm run typecheck).

Here is a short checklist of additional things to keep in mind before submitting:

• Please make sure your pull request description makes it very clear what you're trying to accomplish. If it's a bug fix, please also provide a failing test case (if possible). In either case, please add additional unit test coverage for your changes. :)
• Be sure you have notifications setup so that you'll see my code review responses. (I may ask you to make some adjustments before merging.)

[wuweiweiwu]

Thanks!