Rename OWNERS assignees: to approvers:

They are effectively the same, assignees is deprecated

docs/tutorials/example-tutorial-template.md

@@ -1,6 +1,6 @@
 ---
 title: Example Tutorial Template
-assignees:
+approvers:
 - chenopis
 ---
 
@@ -63,4 +63,4 @@ Here's something interesting about the code you ran in the preceding steps.
 
 {% endcapture %}
 
-{% include templates/tutorial.md %}
+{% include templates/tutorial.md %}