io: unexport ErrBadWriteCount

io: unexport ErrBadWriteCount

It was added in CL 240740 to fix #39978
but without any discussion of the exported API.

The error can still be returned to fix the issue,
without adding new public API to package io.

Also fix the error message to refer to lower-case write
like the other errors in the package.

Change-Id: I134de5eaf3ac903d73913c5cadcde904c5255d79
Reviewed-on: https://go-review.googlesource.com/c/go/+/262877
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>