Correct spelling in DraftEntityMutability docs
Correct spelling in DraftEntityMutability docs

Summary:
**Summary**
Updated a small typo.

**Test Plan**
n/a
Closes https://github.com/facebook/draft-js/pull/1479

Reviewed By: flarnie

Differential Revision: D6212351

Pulled By: flarnie

fbshipit-source-id: 72681385e64e0602ca9919d304dd2f166ef9433f

facebook/draft-js