Fix docs. Change bottomTab title to text (#4288)

I believe there is a type in the docs. The title prop in bottomTab should be text.

docs/docs/top-level-api-migration.md

           }],
           options: {
             bottomTab: {
-              title: 'Tab 1',
+              text: 'Tab 1',
               icon: require('../images/one.png'),
               testID: 'FIRST_TAB_BAR_BUTTON'
             }
           },
           options: {
             bottomTab: {
-              title: 'Tab 2',
+              text: 'Tab 2',
               icon: require('../images/two.png'),
               testID: 'SECOND_TAB_BAR_BUTTON'
             }
 
 In some cases you might need the id of the currently visible screen. This method returns the unique id of the currently visible screen:
 `const visibleScreenInstanceId = await Navigation.getCurrentlyVisibleScreenId()`
-In order to have any use of this method, you'd need to map instanceId to screens your self. -->
+In order to have any use of this method, you'd need to map instanceId to screens your self. -->