fix a typo in navigation doc

Summary: Closes facebook#12921

Differential Revision: D4709474

Pulled By: mkonicek

fbshipit-source-id: 3c7da6ee87d350d52074a05abe168345f936858f

docs/Navigation.md

@@ -56,7 +56,7 @@ class MainScreen extends React.Component {
 }
 ```
 
-React Navigation routers make it easy to override navigation logic or integrate it into redux. Because routers can be nested inside eachother, developers can override navigation logic for one area of the app without making widespread changes.
+React Navigation routers make it easy to override navigation logic or integrate it into redux. Because routers can be nested inside each other, developers can override navigation logic for one area of the app without making widespread changes.
 
 The views in React Navigation use native components and the `Animated` library to deliver 60fps animations that are run on the native thread. Plus, the animations and gestures can be easily customized.