Rename the 2.7.0a1 summary.

I didn't realize these summary files need to be created unique for the
minor releases too

changelogs/.changes.yaml

@@ -94,7 +94,7 @@ releases:
     - user-module-backup-shadow.yaml
     - user_freebsd_always_changed_bugfix.yaml
     - v2.7.0-initial-commit.yaml
-    - v2.7.0_summary.yaml
+    - v2.7.0a1_summary.yaml
     - vars_prompt-allow-prompting-for-play-vars.yaml
     - vault-errors.yaml
     - vultr_modules_renamed.yml

changelogs/fragments/v2.7.0a1_summary.yaml

@@ -1,4 +1,4 @@
 ---
 release_summary: |
-    | Release Date: 2018-10-04
+    | Release Date: 2018-08-31
     | `Porting Guide <https://docs.ansible.com/ansible/devel/porting_guides.html>`__