encoding/binary: improve comment formatting consistency

Use 2 slashes, space, then tab. This is more consistent, and removes
inadvertent leading space.

Change-Id: I383770ed4eb8ac17c78c7ae5675b553d4fb70b1e
Reviewed-on: https://go-review.googlesource.com/46726Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>