a minor correction of an error messages in signal_processing.butter()
a minor correction of an error messages in signal_processing.butter()

carloscanova/elephant,etorre/elephant,Junji110/elephant,mdenker/elephant,btel/elephant,ccluri/elephant,INM-6/elephant,espenhgn/elephant,NeuralEnsemble/elephant,ccluri/elephant,apdavison/elephant,btel/elephant,espenhgn/elephant,JuliaSprenger/elephant,sonjagruen/elephant,INM-6/elephant,NeuralEnsemble/elephant,lzehl/elephant,mdenker/elephant,alperyeg/elephant,neuroelectro/elephant,etorre/elephant,carloscanova/elephant,INM-6/elephant,apdavison/elephant,mdenker/elephant,neuroelectro/elephant,sonjagruen/elephant,lzehl/elephant,dholstein/elephant,NeuralEnsemble/elephant,dholstein/elephant,pietroquaglio/elephant,pietroquaglio/elephant,mczerwinski/elephant,Junji110/elephant,mczerwinski/elephant,alperyeg/elephant