Documentation/arm64: Fix the typo of register names

Documentation/arm64: Fix the typo of register names

The register name 'HWFGWTR_EL2' and 'HWFGRTR_EL2' is wrong, should be 'HFGWTR_EL2' and 'HFGRTR_EL2'.
Find the register description on arm website here,
https://developer.arm.com/documentation/ddi0601/2025-09/AArch64-Registers/HFGWTR-EL2--Hypervisor-Fine-Grained-Write-Trap-Register
https://developer.arm.com/documentation/ddi0601/2025-09/AArch64-Registers/HFGRTR-EL2--Hypervisor-Fine-Grained-Read-Trap-Register?lang=en

Signed-off-by: Zenon Xiu <zenonxiu@outlook.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>