This updates the bundled copy of x/net/http2 to x/net git rev
3470a06c1357df53 for:

http2: fix typo in comment
https://golang.org/cl/44271

http2: fix nil dereference after Read completes with an error
https://golang.org/cl/44330

Fixes #20501

Change-Id: Ie6a5fb092f77044f504f146721dae86299e04011
Reviewed-on: https://go-review.googlesource.com/44331
Run-TryBot: Tom Bergan <tombergan@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>