fix: correct gradle import instructions in get-started.mdx (#2599)

While integrating this on Android, I found that the suggested gradle
import changes weren't correct on the [Get Started
page](https://react-native-iap.dooboolab.com/docs/get-started#with-androidx).

I have made the correction in this PR.

docs/docs/get-started.mdx

@@ -65,11 +65,10 @@ buildscript {
 +    minSdkVersion = 24
 +    kotlinVersion = "1.8.0"
   }
-}
-
-dependencies {
-  ...
-+ classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
+  dependencies {
+    ...
++   classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
+  }
 }
 ```