Update alertsSuppressionRules.json (#26567)

Author: ofermoses

specification/security/resource-manager/Microsoft.Security/preview/2019-01-01-preview/alertsSuppressionRules.json

@@ -246,7 +246,7 @@
         "expirationDateUtc": {
           "type": "string",
           "format": "date-time",
-          "description": "Expiration date of the rule, if value is not provided or provided as null this field will default to the maximum allowed expiration date."
+          "description": "Expiration date of the rule, if value is not provided or provided as null there will no expiration at all"
         },
         "reason": {
           "type": "string",