arm64/sme: Remove spurious 'is' in SME documentation

Just a typographical error.

Reported-by: Edmund Grimley-Evans <edmund.grimley-evans@arm.com>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240124-arm64-sve-sme-doc-v2-4-fe3964fb3c19@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>