There are probably several methods but one is to click on the Crop tool on your toolbar then hover the mouse outside of the shot and near a corner. Your mouse pointer should change to a curved two headed arrow which you can click and drag your shot around to the desired angle (automatically cropping the required amount).
If you have Lightroom there is a tool within Crop which allows you to draw a line along any edge you wish to make straight (looks like a little ruler).
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