doc: deprecation message for Transport.CancelRequest is not correct

[smeriwether]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   N/A
2. What operating system and processor architecture are you using (go env)?
   N/A
3. What did you do?
   If possible, provide a recipe for reproducing the error.
   A complete runnable program is good.
   A link on play.golang.org is best.
   N/A
4. What did you expect to see?
   • Documentation detailing deprecations should not point to code that is also deprecated.
5. What did you see instead?
   • https://golang.org/pkg/net/http/#Transport.CancelRequest, points to another solution which is also deprecated: https://golang.org/pkg/net/http/#Request.

I would not mind submitting a patch for this, simply updating the Transport.CancelRequest deprecation message to match the message in Request.