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: NCatalin Marinas <catalin.marinas@arm.com>

Documentation/arm64/memory.txt

@@ -41,7 +41,7 @@ ffffffbbffff0000	ffffffbcffffffff	  ~2MB		[guard]
 
 ffffffbffc000000	ffffffbfffffffff	  64MB		modules
 
-ffffffc000000000	ffffffffffffffff	 256GB		memory
+ffffffc000000000	ffffffffffffffff	 256GB		kernel logical memory map
 
 
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