testsuite: fix dirty rootfs error message

testsuite: fix dirty rootfs error message

Running a test binary directly on a dirty rootfs results in the
following error message:

        ERR: rootfs is dirty, please run
	'meson compile testsuite/stamp-rootfs' before running this test

Running the command specified by the error message fails:

        $ meson compile testsuite/stamp-rootfs
        ERROR: Can't invoke target `testsuite/stamp-rootfs`:
	target not found

Fix the error message to display the proper target.

Fixes: b85b2a0 ("testsuite: update "make rootfs" error message")
Signed-off-by: Dawid Osuchowski <linux@osuchow.ski>
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>