Update github_disclaimer.md because error in the git adress to be used.

.github/github_disclaimer.md

 # Initial disclaimer
 This project is now tracked on our hosted gitlab server at:
-https://git-sandbox.duniter.org/duniter/gitlab
+https://git.duniter.org/clients/cesium/cesium
 
 The current github repository is a simple clone taken up to date at each push on the main gitlab repository.