board/raspberrypi: fix typo in comment

This fixes a typo by dropping off the spurious x in ensure.

Signed-off-by: Gaël PORTAY <gael.portay@rtone.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/raspberrypi/post-build.sh

@@ -15,5 +15,5 @@ elif [ -d ${TARGET_DIR}/etc/systemd ]; then
        "${TARGET_DIR}/etc/systemd/system/getty.target.wants/getty@tty1.service"
 fi
 
-# exnsure overlays exists for genimage
+# ensure overlays exists for genimage
 mkdir -p "${BINARIES_DIR}/rpi-firmware/overlays"