Fixes missleading comment on getInitialURL

Summary:
The returned value from Linking.getInitialURL is a promise that returns an url.

Can be seen here: f126540519/Libraries/Linking/Linking.js (L175)
Closes https://github.com/facebook/react-native/pull/12851

Differential Revision: D4716084

Pulled By: hramos

fbshipit-source-id: 309881cfb423a5c9a3f9010ae7ca226b63c91599

Libraries/Linking/Linking.js

@@ -33,7 +33,7 @@ const LinkingManager = Platform.OS === 'android' ?
  *
  * ```
  * componentDidMount() {
- *   var url = Linking.getInitialURL().then((url) => {
+ *   Linking.getInitialURL().then((url) => {
  *     if (url) {
  *       console.log('Initial url is: ' + url);
  *     }