doc/go1.14: -d=checkptr is not yet recommended on Windows

Hopefully we'll have the remaining safety violations in the standard
library ironed out by 1.15.

We also fix a minor (but important) typo while we're here.

Updates #34964, #34972.

Change-Id: Ic72fd4d9411b749f8c0cea87e95ab68347009893
Reviewed-on: https://go-review.googlesource.com/c/go/+/214118


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>