Remove action keyword from do-until example

`action:` is old and not needed any more.

docs/docsite/rst/playbooks_loops.rst

@@ -349,8 +349,7 @@ Do-Until Loops
 
 Sometimes you would want to retry a task until a certain condition is met.  Here's an example::
 
-    - action:
+    - shell: /usr/bin/foo
-        shell /usr/bin/foo
       register: result
       until: result.stdout.find("all systems go") != -1
       retries: 5