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

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang