update changelog for fix to JENKINS-8647

changelog.html

@@ -60,6 +60,9 @@ Upcoming changes</a>
 <ul class=image>
   <li class=bug>
     Jenkins was unable to kill/list up native processses on 64bit Mac JVMs.
+  <li class=bug>
+    Many messages about RecordReaper IllegalArgumentException
+    (<a href="http://issues.jenkins-ci.org/browse/JENKINS-8647">issue 8647</a>)
   <li class=rfe>
     Lock down maven plugin versions to shut up m3
     (<a href="http://issues.jenkins-ci.org/browse/JENKINS-7275">issue 7275</a>)