arm64/gcs: Fix documentation for HWCAP

In one of the renumberings of the GCS hwcap a stray reference to HWCAP2 was
left, fix it.

Reported-by: David Spickett <David.Spickett@arm.com>
Fixes: 7058bf87 ("arm64/gcs: Document the ABI for Guarded Control Stacks")
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250124-arm64-gcs-hwcap-doc-v1-1-fa9368b01ca6@kernel.org


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