encoding/binary: make new example a bit more idiomatic

encoding/binary: make new example a bit more idiomatic

Mainly get rid of the weird zero-value struct literal,
but while we're here also group and order things a bit better:
first the reader, then the data, then the call (which takes reader then data).

Change-Id: I901b0661d85d8eaa0807e4482aac66500ca996c7
Reviewed-on: https://go-review.googlesource.com/78118
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>