Fix the branch alias for master

Version was bumped to 2.10-dev in 27023276 but the branch alias was forgotten.

composer.json

@@ -49,7 +49,7 @@
     },
     "extra": {
         "branch-alias": {
-            "dev-master": "2.9.x-dev",
+            "dev-master": "2.10.x-dev",
             "dev-develop": "3.0.x-dev"
         }
     }