fixed spelling mistake (#4313)

Author: Zim Kalinowski

specification/sql/resource-manager/Microsoft.Sql/preview/2017-03-01-preview/longTermRetention.json

@@ -561,7 +561,7 @@
           "type": "string"
         },
         "monthlyRetention": {
-          "description": "The montly retention policy for an LTR backup in an ISO 8601 format.",
+          "description": "The monthly retention policy for an LTR backup in an ISO 8601 format.",
           "type": "string"
         },
         "yearlyRetention": {