fix typo: readonly to readOnly (#16548)

Co-authored-by: Xuyang Cao <[email protected]>

Author: Caoxuyang

specification/appplatform/resource-manager/Microsoft.AppPlatform/preview/2021-09-01-preview/appplatform.json

@@ -4426,8 +4426,8 @@
           "description": "The mount path of the persistent disk.",
           "type": "string"
         },
-        "readonly": {
-          "description": "Indicates whether the persistent disk is a readonly one.",
+        "readOnly": {
+          "description": "Indicates whether the persistent disk is a readOnly one.",
           "type": "boolean"
         },
         "mountOptions": {