Update docs/index.md

Author: fgalan

docs/index.md

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 # Welcome to the FIWARE IoT Agent for OMA Lightweight M2M
 
 [![FIWARE IoT Agents](https://fiware.github.io/catalogue/badges/chapters/iot-agents.svg)](https://www.fiware.org/developers/catalogue/)
-[![](https://fiware.github.io/catalogue/badges/stackoverflow/iot-agents.svg)](https://stackoverflow.com/questions/tagged/fiware+iot)
+[![](https://img.shields.io/badge/tag-fiware+iot-orange.svg?logo=stackoverflow)](https://stackoverflow.com/questions/tagged/fiware+iot)
 
 This Internet of Things Agent is a bridge that can be used to communicate devices using the OMA Lightweight M2M protocol
 and NGSI Context Brokers (like Orion). Lightweight M2M is a lightweight protocol aimed to constrained devices and