Update CHANGELOG for SafetyBuffer slice access

Co-authored-by: Nno-itsbackpack <no-itsbackpack@github.com>

activesupport/CHANGELOG.md

+*   Maintain `html_safe?` on html_safe strings when sliced
+
+        string = "<div>test</div>".html_safe
+        string[-1..1].html_safe? # => true
+
+    *Elom Gomez, Yumin Wong*
+
 *   Add `Array#extract!`.
 
     The method removes and returns the elements for which the block returns a true value.