Fix operationId value (#19975)

Author: alanenriqueo

specification/postgresql/resource-manager/Microsoft.DBforPostgreSQL/preview/2022-01-20-preview/postgresql.json

@@ -512,7 +512,7 @@
         "tags": [
           "Backups"
         ],
-        "operationId": "BackupListByServer",
+        "operationId": "Backups_ListByServer",
         "x-ms-examples": {
           "List backups for a server": {
             "$ref": "./examples/BackupListByServer.json"