Update custom-navigators.md : Add missing variable

[ahmed-khlifi]

In the TabNavigator example , isFocused Variable is using index variable to validate the condition.
However, the index variable is not defined which cause an error of no existing variable.
old example :

state.routes.map((route) => ( // missing index
          <Pressable
            key={route.key}
            onPress={() => {
              const isFocused = state.index === index; // using undefined variable
...........
              }); 
           ..........

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