Update version badge in readme.md

readme.md

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 # R2DBC migration library
-[![Maven Central](https://maven-badges.herokuapp.com/maven-central/name.nkonev.r2dbc-migrate/r2dbc-migrate-spring-boot-starter/badge.svg)](https://central.sonatype.com/namespace/name.nkonev.r2dbc-migrate)
+[![Maven Central](https://img.shields.io/maven-central/v/name.nkonev.r2dbc-migrate/r2dbc-migrate-core)](https://central.sonatype.com/namespace/name.nkonev.r2dbc-migrate)
 [![Build Status](https://github.com/nkonev/r2dbc-migrate/workflows/Java%20CI%20with%20Maven/badge.svg)](https://github.com/nkonev/r2dbc-migrate/actions)
 
 Inspired by [this](https://spring.io/blog/2020/03/12/spring-boot-2-3-0-m3-available-now) announcement. R2DBC [page](https://r2dbc.io/).