Adds missing ref to the 8.16.1 release notes (#6225)

Author: nastasha-solomon

docs/release-notes.asciidoc

@@ -3,6 +3,7 @@
 
 This section summarizes the changes in each release.
 
+* <<release-notes-8.16.1, {elastic-sec} version 8.16.1>>
 * <<release-notes-8.16.0, {elastic-sec} version 8.16.0>>
 * <<release-notes-8.15.4, {elastic-sec} version 8.15.4>>
 * <<release-notes-8.15.3, {elastic-sec} version 8.15.3>>