doc/effectivego: reword confusing sentence

doc/effectivego: reword confusing sentence

For some reason git won't let me write

	doc/effective_go.html: reword confusing sentence

or even

	doc/effective_go: reword confusing sentence

as the subject line for this CL, but that's not important. The
actual CL just rewrites one sentence and adds an option to grep in
the associated example.

Fixes #15875

Change-Id: Iee159ea751caf4b73eacf3dfc86e29032646373f
Reviewed-on: https://go-review.googlesource.com/32110
Reviewed-by: Ian Lance Taylor <iant@golang.org>