One of the lugs into which you have screwed the screws which hold the socket onto the backbox is able to move a little up and down, whereas the other one is 'fixed'.
Hence, if you slightly loosen both the screws (as has been said, best to switch off power to the socket before you do this!), you should find that you can move one side of the socket up and down a little. If you are lucky (in terms of 'which way it needs to be moved') you might then be able to get the socket perfectly horizontal - and then tighten the screws to hold it in that corrected position.
However, as I've said, there's some luck involved. If, for example, the left side of the socket needs to go up (or the right side down) a little, then it is essentially pot luck as to whether the adjustment I have indicated would be able to achieve that (it might only allow you to re-adjust it 'the wrong way', making the situation even worse!).
If you try, let's hope that it is your 'lucky day'
In your picture the Left lug moves up and down about 5mm, therefore you just turn off the power, loosen the 2 screws only about 2 or 3 turns, not to loose place your level on the socket top and then the left side will go up or down usually enough to level the front plate, hold firm then tighten screws a bit, remove level fully tighten and power on again.
Unless the box is a lot out, its usually possible.
it dont look far out, post a pic with the level sitting on top of the socket
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