Un-indent release not blocks in PR template

Port of tektoncd/pipeline#3269

Author: dibyom

.github/pull_request_template.md

@@ -29,19 +29,19 @@ Examples of user facing changes:
 
 For pull requests with a release note:
 
-    ```release-note
-    Your release note here
-    ```
+```release-note
+Your release note here
+```
 
 For pull requests that require additional action from users switching to the new release, include the string "action required" (case insensitive) in the release note:
 
-    ```release-note
-    action required: your release note here
-    ```
+```release-note
+action required: your release note here
+```
 
 For pull requests that don't need to be mentioned at release time, use the `/release-note-none` Prow command to add the `release-note-none` label to the PR. You can also write the string "NONE" as a release note in your PR description:
 
-    ```release-note
-    NONE
-    ```
+```release-note
+NONE
+```
 -->