Update documentation to stop using search path and old import syntax

Summary:
I don't think editing the search path is necessary anymore for notifications since [0.40](https://github.com/facebook/react-native/releases/tag/v0.40.0).

I removed the part that says to edit the search paths, and changed `#import "RCTPushNotificationManager.h"` to `#import <React/RCTPushNotificationManager.h>` in the documentation

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[1]: https://medium.com/martinkonicek/what-is-a-test-plan-8bfc840ec171#.y9lcuqqi9
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Closes https://github.com/facebook/react-native/pull/13700

Differential Revision: D4969728

Pulled By: javache

fbshipit-source-id: d72aee1c5b8578cea27d6b7d45ee7a7f269433fc

Libraries/PushNotificationIOS/PushNotificationIOS.js

@@ -79,14 +79,12 @@ export type PushNotificationEventName = $Enum<{
  *
  * - Add the following to your Project: `node_modules/react-native/Libraries/PushNotificationIOS/RCTPushNotification.xcodeproj`
  * - Add the following to `Link Binary With Libraries`: `libRCTPushNotification.a`
- * - Add the following to your `Header Search Paths`:
- * `$(SRCROOT)/../node_modules/react-native/Libraries/PushNotificationIOS` and set the search to `recursive`
  *
  * Finally, to enable support for `notification` and `register` events you need to augment your AppDelegate.
  *
  * At the top of your `AppDelegate.m`:
  *
- *   `#import "RCTPushNotificationManager.h"`
+ *   `#import <React/RCTPushNotificationManager.h>`
  *
  * And then in your AppDelegate implementation add the following:
  *