Update ContentKeyPolicies.json (Azure#21155)

Author: giakas

specification/mediaservices/resource-manager/Microsoft.Media/Metadata/stable/2022-08-01/ContentKeyPolicies.json

@@ -656,7 +656,7 @@
         "ask": {
           "type": "string",
           "format": "byte",
-          "description": "The key that must be used as FairPlay Application Secret key.",
+          "description": "The key that must be used as FairPlay Application Secret key. This needs to be base64 encoded.",
           "x-nullable": true
         },
         "fairPlayPfxPassword": {