Add links to NGSI-LD API specification

Author: jason-fox

README.md

@@ -45,7 +45,7 @@ git checkout NGSI-LD
 ./services start
 ```
 
-| [![NGSI LD](https://img.shields.io/badge/NGSI-LD-d6604d.svg)](https://www.etsi.org/deliver/etsi_gs/CIM/001_099/009/01.08.01_60/gs_cim009v010801p.pdf) | :books: [Documentation](https://github.com/FIWARE/tutorials.Subscriptions/tree/NGSI-LD) | <img  src="https://cdn.jsdelivr.net/npm/simple-icons@v3/icons/postman.svg" height="15" width="15"> [Postman Collection](https://fiware.github.io/tutorials.Subscriptions/ngsi-ld.html) | ![](https://img.shields.io/github/last-commit/fiware/tutorials.Subscriptions/NGSI-LD)
+| [![NGSI LD](https://img.shields.io/badge/NGSI-LD-d6604d.svg)](https://cim.etsi.org/NGSI-LD/official/0--1.html) | :books: [Documentation](https://github.com/FIWARE/tutorials.Subscriptions/tree/NGSI-LD) | <img  src="https://cdn.jsdelivr.net/npm/simple-icons@v3/icons/postman.svg" height="15" width="15"> [Postman Collection](https://fiware.github.io/tutorials.Subscriptions/ngsi-ld.html) | ![](https://img.shields.io/github/last-commit/fiware/tutorials.Subscriptions/NGSI-LD)
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