Fixed changelog formatting facebook#17481

CHANGELOG.md

@@ -534,7 +534,7 @@ This release was published in a broken state and should be skipped.
 * Fix an issue with `this.state` of different components getting mixed up. ([@sophiebits](https://github.com/sophiebits) in [#12323](https://github.com/facebook/react/pull/12323))
 * Provide a better message when component type is undefined. ([@HeroProtagonist](https://github.com/HeroProtagonist) in [#11966](https://github.com/facebook/react/pull/11966))
 
-## React Test Renderer
+### React Test Renderer
 
 * Fix handling of fragments in `toTree()`. ([@maciej-ka](https://github.com/maciej-ka) in [#12107](https://github.com/facebook/react/pull/12107) and [@gaearon](https://github.com/gaearon) in [#12154](https://github.com/facebook/react/pull/12154))
 * Shallow renderer should assign state to `null` for components that don't set it. ([@jwbay](https://github.com/jwbay) in [#11965](https://github.com/facebook/react/pull/11965))