runtime(doc): add `usr` tag to usr_toc.txt

When typing `:h usr` it redirects to usr_01.txt, but I'd argue
usr_toc.txt is more useful as you can see an overview of all manuals.
When I usr `:h usr` I personally always intend to go to `usr_toc`.

closes: #15779

Signed-off-by: dundargoc <gocdundar@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/usr_toc.txt b/runtime/doc/usr_toc.txt
index 4cb404b..c70c945 100644
--- a/runtime/doc/usr_toc.txt
+++ b/runtime/doc/usr_toc.txt
@@ -1,8 +1,8 @@
-*usr_toc.txt*	For Vim version 9.1.  Last change: 2022 Jun 20
+*usr_toc.txt*	For Vim version 9.1.  Last change: 2024 Oct 01
 
 		     VIM USER MANUAL - by Bram Moolenaar
 
-			      Table Of Contents			*user-manual*
+			      Table Of Contents		       *user-manual* *usr*
 
 ==============================================================================
 Overview ~