runtime: correct parameter name in MCentral_AllocList comment
runtime: correct parameter name in MCentral_AllocList comment

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/14792043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang