documentation: `index.md` appears to have a typo

[hamirmahal]

Summary:

Steps to reproduce:

1. Check out the documentation for react-modal.
2. Jump to the "Using a custom parent node" section.
3. Notice what appears to be a typo in the last paragraph of that section. There's a screenshot below for easier reference.

Expected behavior:

I don't think the character p contributes anything; I think it's just a typo, so it shouldn't be present.

Link to example of issue:

http://reactcommunity.org/react-modal/

Additional notes:

Hope this helps! I looked at the commit history for index.md just for fun and it looks like this was present in the documentation at least since the migration to mkdocs.