Fix wrong hint when deleting a branch successfully from pull request UI (#22673)

Fix #18785

routers/web/repo/pull.go

@@ -1395,7 +1395,7 @@ func CleanUpPullRequest(ctx *context.Context) {
 }
 
 func deleteBranch(ctx *context.Context, pr *issues_model.PullRequest, gitRepo *git.Repository) {
-	fullBranchName := pr.HeadRepo.Owner.Name + "/" + pr.HeadBranch
+	fullBranchName := pr.HeadRepo.FullName() + ":" + pr.HeadBranch
 	if err := repo_service.DeleteBranch(ctx.Doer, pr.HeadRepo, gitRepo, pr.HeadBranch); err != nil {
 		switch {
 		case git.IsErrBranchNotExist(err):