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

[hamirmahal]

Fixes #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.

Changes proposed:

• Updating index.md to remove what I think is a typo.

Upgrade Path (for changed or removed APIs):
I don't think this is applicable.

Acceptance Checklist:

• The commit message follows the guidelines in CONTRIBUTING.md.
• Documentation (README.md) and examples have been updated as needed.
• If this is a code change, a spec testing the functionality has been added.
• If the commit message has [changed] or [removed], there is an upgrade path above.

[coveralls]

Coverage remained the same at 82.632% when pulling 6418b37 on hamirmahal:patch-1 into 94ad567 on reactjs:master.

[diasbruno]

@hamirmahal Thanks!

[hamirmahal]

@diasbruno You're welcome! Happy to help.