The net/http module added support for HTTPS proxies in CL 68550, but the
Transport.Proxy docstring was never updated to reflect this. This (doc-only)
update adds "https" to the list of supported schemes.

Change-Id: I0570fcdae8232bb42d52c4dd739dd09ee8dfd612
Reviewed-on: https://go-review.googlesource.com/126495Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>