s/each are/each is (#713)

"Each" here is singular so the verb that follows it must also be singular (i.e. "is" not "are"). See:
 
- https://english.stackexchange.com/questions/24846/how-does-each-change-are-to-is
- https://english.stackexchange.com/questions/12387/each-with-plural-or-singular-verb

content/docs/integrating-with-other-libraries.md

@@ -10,7 +10,7 @@ React can be used in any web application. It can be embedded in other applicatio
 
 React is unaware of changes made to the DOM outside of React. It determines updates based on its own internal representation, and if the same DOM nodes are manipulated by another library, React gets confused and has no way to recover.
 
-This does not mean it is impossible or even necessarily difficult to combine React with other ways of affecting the DOM, you just have to be mindful of what each are doing.
+This does not mean it is impossible or even necessarily difficult to combine React with other ways of affecting the DOM, you just have to be mindful of what each is doing.
 
 The easiest way to avoid conflicts is to prevent the React component from updating. You can do this by rendering elements that React has no reason to update, like an empty `<div />`.