update changelog entry

Author: cognifloyd

CHANGELOG.rst

@@ -16,6 +16,7 @@ Added
   to pants' use of PEX lockfiles. This is not a user-facing addition.
   #5778 #5789 #5817 #5795 #5830 #5833 #5834 #5841 #5840 #5838 #5842 #5837 #5849 #5850
   #5846 #5853 #5848 #5847 #5858 #5857 #5860 #5868 #5871 #5864 #5874 #5884 #5893 #5891
+  #5897
   Contributed by @cognifloyd
 
 * Added a joint index to solve the problem of slow mongo queries for scheduled executions. #5805