sources.json: replace refs with branches

refs are the old way of referring to what niv tracks for updates
niv actively encourages to switch to branches or tags so this is done
here

nix/sources.json

@@ -1,6 +1,6 @@
 {
     "haccmap": {
-        "ref": "master",
+        "branch": "master",
         "repo": "https://gitlab.infra4future.de/hacc/haccspace-rc3-map",
         "rev": "3b99ee7e5bb2f27044784d03572da77d39cb2427",
         "type": "git"
@@ -89,7 +89,7 @@
         "type": "git"
     },
     "workadventure": {
-        "ref": "master",
+        "branch": "master",
         "repo": "https://stuebinm.eu/git/workadventure-nix",
         "rev": "cbe9196fba75be98569d7504e7c1d091bcbf053e",
         "type": "git"