chore: Update package README (#2601)

Update README.md

yarn bootstrap needs package.json which is in package directory.

package/example/README.md

@@ -30,7 +30,7 @@ To try the playground out for yourself, run the following commands:
 
 ```sh
 git clone https://github.com/mrousavy/react-native-vision-camera
-cd react-native-vision-camera
+cd react-native-vision-camera/package
 yarn bootstrap
 ```