Docs/admin-guide/mm/damon/start: fix a wrong link

The 'Getting Started' of DAMON is providing a link to DAMON's user
interface document while saying about its user space tool's detailed
usages.  This fixes the link.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: SeongJae Park <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Peter Xu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: sjp38

Documentation/admin-guide/mm/damon/start.rst

@@ -6,7 +6,9 @@ Getting Started
 
 This document briefly describes how you can use DAMON by demonstrating its
 default user space tool.  Please note that this document describes only a part
-of its features for brevity.  Please refer to :doc:`usage` for more details.
+of its features for brevity.  Please refer to the usage `doc
+<https://github.com/awslabs/damo/blob/next/USAGE.md>`_ of the tool for more
+details.
 
 
 TL; DR