ssh: rename ExampleHostKeyCheck to ExampleClientConfig_HostKeyCallback
ssh: rename ExampleHostKeyCheck to ExampleClientConfig_HostKeyCallback

Change-Id: I6d0f3396f06871286e3d48ec5af38770663dbed2
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/171678
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/crypto,golang/crypto,golang/crypto