Add brackets to ImageURL in user-input-plug.component.html

The paramenter `image_url` in `openhtf.plugs.user_input.UserInput.promt()` doesn't seem to have any effect. After adding brackets to the file mentioned in the title, the given image shows up just fine.

I already opened a PR, but closed it again as I don't want to log in with a Google account for such a small change. If somebody else wants to do that, be my guest. Here's my (now closed) pull request: #1157