Fix warning in README

Author: VolkerHartmann

README.md

@@ -4,10 +4,10 @@
 [![Coverage Status](https://coveralls.io/repos/github/kit-data-manager/indexing-service/badge.svg?branch=master)](https://coveralls.io/github/kit-data-manager/indexing-service?branch=master)
 ![License](https://img.shields.io/github/license/kit-data-manager/indexing-service.svg)
 
-:::warning
+:warning:
 Not fully tested yet!
 For mapping documents only Gemma is available currently!
-:::
+
 A standalone service which receives messages about changes in metadata documents maps them to a common format and ingests them into elasticsearch.
 
 ![Visualization of use case structure.](use-case.drawio.svg)