Per Zendesk ticket #180630, we need to update our docs to reflect that...

Per Zendesk ticket #180630, we need to update our docs to reflect that iteration burndown and burnup charts didn't make the 13.5 milestone and were instead added in 13.6.

doc/user/group/iterations/index.md

@@ -144,7 +144,7 @@ To view an iteration report, go to the iterations list page and select an iterat
 
 ### Iteration burndown and burnup charts
 
-> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/222750) in GitLab 13.5.
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/222750) in GitLab 13.6.
 > - [Feature flag removed](https://gitlab.com/gitlab-org/gitlab/-/issues/269972) in GitLab 13.7.
 
 The iteration report includes [burndown and burnup charts](../../project/milestones/burndown_and_burnup_charts.md),