[grenoble] Updated latitude & longitude values.

data/grid5000/sites/grenoble/grenoble.json

 {
   "description": "",
   "email_contact": null,
-  "latitude": null,
+  "latitude": 45.1833,
   "location": "Grenoble, France",
-  "longitude": null,
+  "longitude": 5.7167,
   "name": "Grenoble",
   "security_contact": null,
   "sys_admin_contact": null,