scripts: check_extable: fix typo in user error message

Fix typo ("and" should be "an") in an error message.

Link: https://lkml.kernel.org/r/20210727002943.29774-1-rdunlap@infradead.org


Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>