[doc] Make links more precise

With a more recent Hugo version (v0.65.3/extended) using the current
links results in a "REF_NOT_FOUND" error.
Preparation for possible updates to Hugo.

Signed-off-by: Tobias Wölfel <tobias.woelfel@mailbox.org>
diff --git a/doc/ug/install_instructions/index.md b/doc/ug/install_instructions/index.md
index 3031d9f..6be0444 100644
--- a/doc/ug/install_instructions/index.md
+++ b/doc/ug/install_instructions/index.md
@@ -38,7 +38,7 @@
 
 ### Clone repository
 
-If you intend to contribute back to OpenTitan you will want your own fork of the repository on GitHub and to work using that, see the [notes for using GitHub]({{< relref "github_notes.md" >}}).
+If you intend to contribute back to OpenTitan you will want your own fork of the repository on GitHub and to work using that, see the [notes for using GitHub]({{< relref "doc/ug/github_notes.md" >}}).
 Otherwise make a simple clone of the main OpenTitan repository.
 
 ```console