At that focal length, with that lens the only way to fix it "in camera" is to put the horizon line right through the centre (boring) so software is your only hope.
Here is a "quick and Dirty" Paintshop fix, used pinch to fix the distortion and then did a very quick redraw od the horizon. I deleted the picture from my computer as soon as it was completed. This was just a 20 minute job to show how it could be done, it would take some practice to get it right.
If you have already viewed this image click refresh on your browser to see the final version.