doc: replace "private" with "unexported" in Effective Go.
doc: replace "private" with "unexported" in Effective Go.

R=r
CC=golang-codereviews
https://codereview.appspot.com/53720043

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