Remove the wheel and then the derailleur, keeping the derailleur oriented as it is now. Carefully rotate the derailleur clockwise (parallel to the wheel) one full turn and then mount back on the bike.

OR

Remove the wheel and the bolt to the bottom pulley, and remove the chain from the pulley cage, leaving the derailleur mounted. Rotate the pulley assembly one turn counter-cliockwise, then run the chain in front of the top pulley and in back of the bottom one as you mount the bottom pulley back in the cage.

That will retension the spring properly. From now on be sure to only loosen the axle nuts - not the bolt that holds the derailleur onto the dropout - and you won't get the chain in a not.