Update loop_nested.yml

[giner]

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])