chore(docs): Fixed broken link in Custom-Android.md (#676)

"Native UI Components" needs to link to the main React Native site, not within this repo.

docs/Custom-Android.md

 While the built-in web view has a lot of features, it is not possible to handle every use-case in React Native. You can, however, extend the web view with native code without forking React Native or duplicating all the existing web view code.
 
-Before you do this, you should be familiar with the concepts in [native UI components](native-components-android). You should also familiarise yourself with the [native code for web views](https://github.com/facebook/react-native/blob/master/ReactAndroid/src/main/java/com/facebook/react/views/webview/ReactWebViewManager.java), as you will have to use this as a reference when implementing new features—although a deep understanding is not required.
+Before you do this, you should be familiar with the concepts in [native UI components](https://facebook.github.io/react-native/docs/native-components-android). You should also familiarise yourself with the [native code for web views](https://github.com/facebook/react-native/blob/master/ReactAndroid/src/main/java/com/facebook/react/views/webview/ReactWebViewManager.java), as you will have to use this as a reference when implementing new features—although a deep understanding is not required.
 
 ## Native Code