Update custom-navigators.md : Add missing variable (react-navigation#1412)

In the TabNavigator example , isFocused Variable is using "index" variable to validate the condiation. However, in state.routes.map((route)...) the index variable is not defined which cause an error of no existing variable.

Author: ahmed-khlifi

versioned_docs/version-7.x/custom-navigators.md

@@ -81,7 +81,7 @@ function TabNavigator({
   return (
     <NavigationContent>
       <View style={[{ flexDirection: 'row' }, tabBarStyle]}>
-        {state.routes.map((route) => (
+        {state.routes.map((route, index) => (
           <Pressable
             key={route.key}
             onPress={() => {