fix: display correct sdk version in readme header

The pom module is deprecated and is no longer being published. Instead,
the version badge must refer to an active SDK module. Core is the most
widely used, so it is selected.

Author: jamesonwilliams

README.md

@@ -2,7 +2,7 @@
 
 [![DiscordChat](https://img.shields.io/discord/308323056592486420?logo=discord)](https://discord.gg/jWVbPfC)
 [![GitHub release](https://img.shields.io/github/release/aws-amplify/aws-sdk-android.svg)](https://github.com/aws-amplify/aws-sdk-android/releases)
-[![Maven Central](https://img.shields.io/maven-central/v/com.amazonaws/aws-android-sdk-pom.svg)](https://search.maven.org/search?q=a:aws-android-sdk-pom)
+[![Maven Central](https://img.shields.io/maven-central/v/com.amazonaws/aws-android-sdk-core.svg)](https://search.maven.org/search?q=a:aws-android-sdk-core)
 [![CircleCI](https://circleci.com/gh/aws-amplify/aws-sdk-android.svg?style=svg)](https://circleci.com/gh/aws-amplify/aws-sdk-android)
 
 For new projects, we recommend interacting with AWS using the [Amplify Framework](https://docs.amplify.aws/start/q/integration/android).