Fix an issue “ServicePrincipalCredentialEnum” should be string rather than object (#16322)

Author: Jingshu923

specification/synapse/data-plane/Microsoft.Synapse/preview/2021-06-01-preview/entityTypes/LinkedService.json

@@ -764,7 +764,7 @@
         "ServicePrincipalKey",
         "ServicePrincipalCert"
       ],
-      "type": "object",
+      "type": "string",
       "description": "The service principal credential type to use in Server-To-Server authentication. 'ServicePrincipalKey' for key/secret, 'ServicePrincipalCert' for certificate. Type: string (or Expression with resultType string)."
     },
     "CommonDataServiceForAppsLinkedService": {

specification/synapse/data-plane/Microsoft.Synapse/stable/2020-12-01/entityTypes/LinkedService.json

@@ -764,7 +764,7 @@
         "ServicePrincipalKey",
         "ServicePrincipalCert"
       ],
-      "type": "object",
+      "type": "string",
       "description": "The service principal credential type to use in Server-To-Server authentication. 'ServicePrincipalKey' for key/secret, 'ServicePrincipalCert' for certificate. Type: string (or Expression with resultType string)."
     },
     "CommonDataServiceForAppsLinkedService": {