Fix Android example

Fix invalid import.

example/android/app/src/main/java/com/example/MainActivity.java

 package com.example;
 
 
-import com.reactnativenavigation.activities.RctActivity;
+import com.reactnativenavigation.activities.RootActivity;
 
-public class MainActivity extends RctActivity {
+public class MainActivity extends RootActivity {
 
     @Override
     protected String getMainComponentName() {