correction of a few documentation typo (#563)

docs/api/routers/Routers.md

@@ -105,8 +105,8 @@ const MyApp = StackNavigator({
 }, {
   initialRouteName: 'Home',
 })
-MyApp.router = {
-  ...MyApp.router,
+const previousGetActionForPathAndParams = MyApp.router.getActionForPathAndParams
+Object.assign(MyApp.router, {
   getActionForPathAndParams(path, params) {
     if (
       path === 'my/custom/path' &&
@@ -123,7 +123,7 @@ MyApp.router = {
         }),
       });
     }
-    return MyApp.router.getStateForAction(action, state);
+    return previousGetActionForPathAndParams(path, params);
   },
 };
 ```

docs/api/routers/RoutersAPI.md

@@ -4,7 +4,7 @@ You can make your own router by building an object with the following functions:
 
 ```js
 const MyRouter = {
-  getStateForAction: (action) => ({}),
+  getStateForAction: (action, state) => ({}),
   getActionForPathAndParams: (path, params) => null,
   getPathAndParamsForState: (state) => null,
   getComponentForState: (state) => MyScreen,