cargo_common.bbclass: Fix typos

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit c7a92180)
Signed-off-by: Steve Sakoman <steve@sakoman.com>