Update README.md

Author: satta

README.md

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 # balboa-backend-java
 
 ![Java CI](https://github.com/satta/balboa-backend-java/workflows/Java%20CI/badge.svg)
-[![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.github.satta/balboa-backend-java/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.github.satta/balboa-backend-java)
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.github.satta/balboa-backend-java/badge.svg?style=flat)](https://maven-badges.herokuapp.com/maven-central/com.github.satta/balboa-backend-java)
 
 This is a balboa backend connector for Java. It takes care of the boring msgpack
 deserialization and validation, and provides a pluggable interface for backends