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.cnSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/arm64/tagged-address-abi.rst

@@ -49,7 +49,7 @@ how the user addresses are used by the kernel:
 
    - ``brk()``, ``mmap()`` and the ``new_address`` argument to
      ``mremap()`` as these have the potential to alias with existing
-      user addresses.
+     user addresses.
 
      NOTE: This behaviour changed in v5.6 and so some earlier kernels may
      incorrectly accept valid tagged pointers for the ``brk()``,