No need to say "by default" because there is no alternative and no way
to override. Always HTTP/2.0 is officially spelled HTTP/2 these days.

Fixes #13985 harder

Change-Id: Ib1ec03cec171ca865342b8e7452cd4c707d7b770
Reviewed-on: https://go-review.googlesource.com/18720Reviewed-by: Rob Pike <r@golang.org>