Improve types, fix typo
Improve types, fix typo

Summary: I made `DraftBlockRenderConfig` exact, which exposed a bug in `OCDevUIMarkdownRenderer.react` (a typo). Fixed it.

Differential Revision: D23976100

fbshipit-source-id: 5a0abd08ee3b00edb55f6b2597ab6c28739bba50

facebook/draft-js