Now you can flatten the image back down to a background by clicking on Flatten Image in the Layers menu, and get rid of the guide line using Clear Guides in the View menu.
All that remains is to trim off the untidy angled edged by using the crop tool. Drag the edges of the crop frame as close as you can to the edges to keep as much of the original image as possible, or crop it smaller to improve the composition.
This method is a bit more long-winded than the simple one-click rotate tools found in most other editing programs, but it does allow you more control over the result, which is what Photoshop is all about. And here's the result, which is a big improvement.