[fixed] extra 'p' character in index.md
[fixed] extra 'p' character in index.md

Closes #830 by removing what I think is a typo: the character 'p' at the beginning of the "Using a custom parent node" on react-modal's documentation page.
reactjs/react-modal,diasbruno/react-modal,diasbruno/react-modal,reactjs/react-modal,diasbruno/react-modal,reactjs/react-modal