time: update doc string for Error method Fixes issue 2512.
time: update doc string for Error method
Fixes issue 2512.

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/5449057

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