net/http: update link to chrome documentation on connection management

The previous link at
https://insouciant.org/tech/connection-management-in-chromium/ is no
longer accessible. This CL changes it to
https://www.chromium.org/developers/design-documents/network-stack#TOC-Connection-Management.

Fixes #38885.

Change-Id: I0881e72fe0c099294ab137b5e2d0c3f5763978f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/232357


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