Fix some typos and update db transaction demo in backend guideline (#21322)

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

docs/content/doc/developers/guidelines-backend.md

@@ -67,22 +67,18 @@ Some actions should allow for rollback when database record insertion/update/del
 So services must be allowed to create a database transaction. Here is some example,
 
 ```go
-// servcies/repository/repo.go
-func CreateXXXX() error {\
-  ctx, committer, err := db.TxContext()
-	if err != nil {
-		return err
-	}
-	defer committer.Close()
-
-  // do something, if return err, it will rollback automatically when `committer.Close()` is invoked.
+// services/repository/repository.go
+func CreateXXXX() error {
+    return db.WithTx(func(ctx context.Context) error {
+        e := db.GetEngine(ctx)
+        // do something, if err is returned, it will rollback automatically
         if err := issues.UpdateIssue(ctx, repoID); err != nil {
             // ...
+            return err
         }
-
-  // ......
-
-  return committer.Commit()
+        // ...
+        return nil
+    })
 }
 ```
 
@@ -94,14 +90,14 @@ If the function will be used in the transaction, just let `context.Context` as t
 func UpdateIssue(ctx context.Context, repoID int64) error {
     e := db.GetEngine(ctx)
 
-    // ......
+    // ...
 }
 ```
 
 ### Package Name
 
 For the top level package, use a plural as package name, i.e. `services`, `models`, for sub packages, use singular,
-i.e. `servcies/user`, `models/repository`.
+i.e. `services/user`, `models/repository`.
 
 ### Import Alias