internal/unsafeheader: fix typos in comments
internal/unsafeheader: fix typos in comments

The typos in the package godoc comment were pointed out in CL 231223
which added similar functionality to the main Go repository. Also
correct the name in the godoc comment for type String.

Change-Id: I69910f3e24b1222190e1adf7fc3d425f0006f96e
Reviewed-on: https://go-review.googlesource.com/c/sys/+/231517
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/sys,multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,golang/sys