mm: Fix a dangling Documentation/arm64 reference

The arm64 documentation has moved under Documentation/arch/.  Fix up a
reference in mm/mremap.c to match.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

mm/mremap.c

@@ -914,7 +914,8 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 	 * mapping address intact. A non-zero tag will cause the subsequent
 	 * range checks to reject the address as invalid.
 	 *
-	 * See Documentation/arm64/tagged-address-abi.rst for more information.
+	 * See Documentation/arch/arm64/tagged-address-abi.rst for more
+	 * information.
 	 */
 	addr = untagged_addr(addr);