Update README.md

Author: BorjaOuterelo

README.md

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 [![GitHub license](https://img.shields.io/github/license/microROS/micro-ROS-Agent.svg)](https://github.com/microROS/micro-ROS-Agent)
 [![GitHub release](https://img.shields.io/github/release/microROS/micro-ROS-Agent.svg)](https://github.com/microROS/micro-ROS-Agent/releases)
-[![GitHub (pre-)release](https://img.shields.io/github/release-pre/microROS/micro-ROS-Agent.svg?label=pre-release)](https://github.com/microROS/micro-ROS-Agent/releases)
 
 ROS 2 package using Micro XRCE-DDS Agent.