Documentation/arm64/memory.txt: fix typo

There is no swapper_pgd_dir, it meant swapper_pg_dir.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/arm64/memory.txt

diff --git a/Documentation/arm64/memory.txt b/Documentation/arm64/memory.txt
index 344e85c..d7273a5 100644
--- a/Documentation/arm64/memory.txt
+++ b/Documentation/arm64/memory.txt
@@ -17,7 +17,7 @@
 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.