ssh: fix format string in client_test.go

Change-Id: I92c3916b0b5628dc2079af82202d9bfef032c708
Reviewed-on: https://go-review.googlesource.com/39430
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
1 file changed