routerrpc: fix estimator model field

This commit fixes the capitalization of the estimator's model field.

lnrpc/routerrpc/router.swagger.json

@@ -1475,7 +1475,7 @@
           "format": "uint64",
           "description": "The minimum time that must have passed since the previously recorded failure\nbefore we raise the failure amount."
         },
-        "Model": {
+        "model": {
           "$ref": "#/definitions/MissionControlConfigProbabilityModel",
           "description": "ProbabilityModel defines which probability estimator should be used in\npathfinding."
         },