555 Monostable Timer delay
If you have reduced Rt,Ct and the pulse is still too long, consider that the trigger input signal may also be too long. (this circuit is designed to stretch an input pulse, not shorten it!) In the circuit you linked to, that is controlled by C1 and R4 (though their instructions suggest it is controlled by C1 and R3)

Reduce C1 to 10nf and try again. Reducing C1 too far will make the triggering unreliable.