runtime(doc): Add missing garbagecollect() hypertext link

closes: #16975

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/testing.txt b/runtime/doc/testing.txt
index 01e98cb..e6f4590 100644
--- a/runtime/doc/testing.txt
+++ b/runtime/doc/testing.txt
@@ -1,4 +1,4 @@
-*testing.txt*	For Vim version 9.1.  Last change: 2025 Mar 23
+*testing.txt*	For Vim version 9.1.  Last change: 2025 Mar 25
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -68,7 +68,7 @@
 
 
 test_garbagecollect_now()			 *test_garbagecollect_now()*
-		Like garbagecollect(), but executed right away.  This must
+		Like |garbagecollect()|, but executed right away.  This must
 		only be called directly to avoid any structure to exist
 		internally, and |v:testing| must have been set before calling
 		any function.   *E1142*