Update loop_nested.yml

Fix the example of referencing a variable for looping with `with_nested`.

Before:
```
changed: [localhost] => (item=[u'listvar1', 1])
changed: [localhost] => (item=[u'listvar1', 2])
changed: [localhost] => (item=[u'listvar1', 3])
```

After:
```
changed: [localhost] => (item=[u'a', 1])
changed: [localhost] => (item=[u'a', 2])
changed: [localhost] => (item=[u'a', 3])
changed: [localhost] => (item=[u'b', 1])
changed: [localhost] => (item=[u'b', 2])
changed: [localhost] => (item=[u'b', 3])
changed: [localhost] => (item=[u'c', 1])
changed: [localhost] => (item=[u'c', 2])
changed: [localhost] => (item=[u'c', 3])
```

Author: giner

language_features/loop_nested.yml

@@ -20,5 +20,5 @@
   tasks:
     - shell: echo "nested test a={{ item[0] }} b={{ item[1] }}"
       with_nested:
-        - listvar1
+        - "{{ listvar1 }}"
         - [ 1, 2, 3 ]