fixed isMigrated to boolean (#12647)

Author: yashmuel

specification/monitor/resource-manager/Microsoft.Insights/stable/2018-03-01/metricAlert_API.json

@@ -551,7 +551,7 @@
           "description": "Last time the rule was updated in ISO8601 format."
         },
         "isMigrated": {
-          "type": "string",
+          "type": "boolean",
           "description": "the value indicating whether this alert rule is migrated.",
           "readOnly": true
         }
@@ -620,7 +620,7 @@
           "description": "Last time the rule was updated in ISO8601 format."
         },
         "isMigrated": {
-          "type": "string",
+          "type": "boolean",
           "description": "the value indicating whether this alert rule is migrated.",
           "readOnly": true
         }