crypto/ssh: fix typo in error string.

cound -> could
Also change fmt.Errorf -> errors.New for consistency.

Change-Id: I0b6900a76b70cd37ab2e8d61327868b46366ec64
Reviewed-on: https://go-review.googlesource.com/11235
Reviewed-by: Adam Langley <agl@golang.org>

ssh/session.go

@@ -339,7 +339,7 @@ func (s *Session) Shell() error {
 
 	ok, err := s.ch.SendRequest("shell", true, nil)
 	if err == nil && !ok {
-		return fmt.Errorf("ssh: cound not start shell")
+		return errors.New("ssh: could not start shell")
 	}
 	if err != nil {
 		return err