docs/arm64: Fix typo'd #define in sve.rst

docs/arm64: Fix typo'd #define in sve.rst

sve.rst describes a flag PR_SVE_SET_VL_INHERIT for the
PR_SVE_SET_VL prctl, but there is no flag of this name.  The flag
is shared between the _GET and _SET calls, so the _SET prefix was
dropped, giving the name PR_SVE_VL_INHERIT in the headers.

Fix it.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Link: https://lore.kernel.org/r/1591808590-20210-2-git-send-email-Dave.Martin@arm.com
Signed-off-by: Will Deacon <will@kernel.org>