Documentation: Fixes a word in Documentation/arm64/memory.txt

Fixes a term in Documentation/arm64/memory.txt
The modification is based on Catalin Marinas <catalin.marinas@arm.com>'s
suggestion:
    "memory" in "ffffffc000000000 ffffffffffffffff 256GB memory"
    should  be treated as "kernel logical memory map".

Signed-off-by: Fu Wei <tekkamanninja@gmail.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>