Fix React Native README startup wording

Author: niemyjski

README.md

@@ -38,7 +38,7 @@ try {
 ## Node
 
 You can install the npm package via `npm install @exceptionless/node --save`.
-Next, you just need to call startup during your apps startup to automatically
+Next, you just need to call startup during your app's startup to automatically
 capture unhandled errors.
 
 ```js