accel/tcg: Fix typo in translator_io_start() description

Missed while reviewing commit dfd1b812
("accel/tcg: Introduce translator_io_start").

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>