Update the release schedule (#54207)

We've slipped an additional week.

docs/docsite/rst/roadmap/ROADMAP_2_8.rst

@@ -13,17 +13,17 @@ Expected
 
 PRs must be raised well in advance of the dates below to have a chance of being included in this Ansible release.
 
-- 2019-03-21 Alpha 1 **Core freeze**
-
+- 2019-03-28 Alpha 1 **Core freeze**
   No new features to ``support:core`` code.
   Includes no new options to existing Core modules
-- 2019-03-28 Beta 1 **Feature freeze**
 
+- 2019-04-04 Beta 1 **Feature freeze**
   No new functionality (including modules/plugins) to any code
-- 2019-04-11 Release Candidate 1
-- 2019-04-18 Release Candidate 2 (if needed)
-- 2019-04-25 Release Candidate 3 (if needed)
-- 2019-05-02 Release
+
+- 2019-04-18 Release Candidate 1
+- 2019-04-25 Release Candidate 2 (if needed)
+- 2019-05-02 Release Candidate 3 (if needed)
+- 2019-05-09 Release