Update docs (#3196)

Outdated onAppLaunched part is changed

I just started to a project with v2 and I realized this part of the documentation was wrong. I searched a bit and find this 2 commits: 

104e3f97fd

1d2680c5aa

So, that part is probably forgotten

docs/docs/Usage.md

 ```
 
 
-### onAppLaunched(callback)
+### registerAppLaunchedListener(callback)
 This event is called once the app is launched. It's where you will initialize the app with the layout you want, via the SetRoot command. This creates the native layout hierarchy, loading React components into the `component` by name. 
 
-Afterwards, the app is ready for user interaction. (Common gotcha: Be sure not to run setRoot before onAppLaunched() has fired!)
+Afterwards, the app is ready for user interaction. (Common gotcha: Be sure not to run setRoot before registerAppLaunchedListener() has fired!)
 
 ```js
-Navigation.events().onAppLaunched(() => {
+Navigation.events().registerAppLaunchedListener(() => {
   Navigation.setRoot({
     component: {
       name: 'navigation.playground.WelcomeScreen'