Documentation: wrong file reference for build.gradle

on [Installation page](https://cdiddy77.github.io/react-native-mediapipe/docs/intro)

![image](https://github.com/cdiddy77/react-native-mediapipe/assets/4615465/23c609f8-2d6c-4872-b5da-df3ce682a0d6)

it's not `gradle/build.gradle` (should be back-tick quotes) its `android/build.gradle`