arm64/booting: Add missing colon to FA64 entry

The booting documentation for FEAT_SME_FA64 is missing a colon that is
normally present, add it.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221107171913.294565-1-broonie@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>