ssh: rename ExampleHostKeyCheck to ExampleClientConfig_HostKeyCallback

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

ssh/example_test.go

diff --git a/ssh/example_test.go b/ssh/example_test.go
index b910c7b..4fe26b0 100644
--- a/ssh/example_test.go
+++ b/ssh/example_test.go
@@ -140,7 +140,7 @@
 	}
 }
 
-func ExampleHostKeyCheck() {
+func ExampleClientConfig_HostKeyCallback() {
 	// Every client must provide a host key check.  Here is a
 	// simple-minded parse of OpenSSH's known_hosts file
 	host := "hostname"