docs/arm64: delete a space from tagged-address-abi

Since e71e2ace

("userfaultfd: do not untag user pointers") which
introduced a warning:

linux/Documentation/arm64/tagged-address-abi.rst:52: WARNING: Unexpected indentation.

Let's fix it.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/20211209091922.560979-1-siyanteng@loongson.cn
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>