arch/Kconfig: update a broken file reference

Commit adab66b7 ("Revert: "ring-buffer: Remove HAVE_64BIT_ALIGNED_ACCESS"")
added the config HAVE_64BIT_ALIGNED_ACCESS back into arch/Kconfig with this
revert. In the meantime, commit c9b54d6f ("docs: move other kAPI
documents to core-api") changed ./Documentation/unaligned-memory-access.txt
to ./Documentation/core-api/unaligned-memory-access.rst.

Fortunately, ./scripts/documentation-file-ref-check detects this and warns
about this broken reference.

Update the file reference in arch/Kconfig.

Fixes: adab66b7 ("Revert: "ring-buffer: Remove HAVE_64BIT_ALIGNED_ACCESS"")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20210119095326.13896-1-lukas.bulwahn@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

arch/Kconfig

@@ -156,8 +156,8 @@ config HAVE_64BIT_ALIGNED_ACCESS
 	  accesses are required to be 64 bit aligned in this way even
 	  though it is not a 64 bit architecture.
 
-	  See Documentation/unaligned-memory-access.txt for more
-	  information on the topic of unaligned memory accesses.
+	  See Documentation/core-api/unaligned-memory-access.rst for
+	  more information on the topic of unaligned memory accesses.
 
 config HAVE_EFFICIENT_UNALIGNED_ACCESS
 	bool