[nancy] Updated latitude & longitude values.

data/grid5000/sites/nancy/nancy.json

 {
   "description": "",
-  "email_contact": null,
-  "latitude": null,
+  "latitude": 48.7,
   "location": "Nancy, France",
-  "longitude": null,
+  "longitude": 6.2,
   "name": "Nancy",
   "security_contact": null,
   "sys_admin_contact": null,