ARM: 7381/1: nommu: fix typo in mm/Kconfig

The description for the CPU_HIGH_VECTOR Kconfig option for nommu builds
doesn't make any sense.

This patch fixes up the trivial grammatical error.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>