cmd/godoc: fix BUGS heading in text mode
cmd/godoc: fix BUGS heading in text mode

All the headings are CAPS not Title.

Fixes issue 4843.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7792043

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