Documentation/arm64/memory.txt: fix typo

There is no swapper_pgd_dir, it meant swapper_pg_dir.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>

Documentation/arm64/memory.txt

@@ -17,7 +17,7 @@ User addresses have bits 63:48 set to 0 while the kernel addresses have
 the same bits set to 1. TTBRx selection is given by bit 63 of the
 virtual address. The swapper_pg_dir contains only kernel (global)
 mappings while the user pgd contains only user (non-global) mappings.
-The swapper_pgd_dir address is written to TTBR1 and never written to
+The swapper_pg_dir address is written to TTBR1 and never written to
 TTBR0.