ssh: fix documentation on ServerConfig.PublicKeyCallback

Fix leftovers from the old days to match today's method signature.

Fixes golang/go#21325

Change-Id: Iac5d49bc7cf9e80d07e21148b5e6e1b9bbb59d74
Reviewed-on: https://go-review.googlesource.com/53472
Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ssh/server.go

@@ -67,7 +67,7 @@ type ServerConfig struct {
 	PasswordCallback func(conn ConnMetadata, password []byte) (*Permissions, error)
 
 	// PublicKeyCallback, if non-nil, is called when a client
-	// offers a public key for authentication. It must return true
+	// offers a public key for authentication. It must return a nil error
 	// if the given public key can be used to authenticate the
 	// given user. For example, see CertChecker.Authenticate. A
 	// call to this function does not guarantee that the key