Fixing typo in Mixins should not be used sentence

Author: coreyshirk

README.md

@@ -1731,7 +1731,7 @@
 
 84. ### What are React Mixins?
 
- *Mixins* are a way to totally separate components to have a common functionality. Mixins are **should not be used** and can be replaced with *higher-order components* or *decorators*.
+ *Mixins* are a way to totally separate components to have a common functionality. Mixins **should not be used** and can be replaced with *higher-order components* or *decorators*.
 
  One of the most commonly used mixins is `PureRenderMixin`. You might be using it in some components to prevent unnecessary re-renders when the props and state are shallowly equal to the previous props and state: