spec: shorten heading for complex, real, imag
spec: shorten heading for complex, real, imag

In the current TOC it sticks out quite a bit.

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/5705053

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