net/http: document Transport.Proxy's https support

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>

src/net/http/transport.go

@@ -121,8 +121,8 @@ type Transport struct {
 	// Request. If the function returns a non-nil error, the
 	// request is aborted with the provided error.
 	//
-	// The proxy type is determined by the URL scheme. "http"
-	// and "socks5" are supported. If the scheme is empty,
+	// The proxy type is determined by the URL scheme. "http",
+	// "https", and "socks5" are supported. If the scheme is empty,
 	// "http" is assumed.
 	//
 	// If Proxy is nil or returns a nil *URL, no proxy is used.