Update preference.asciidoc
Update preference.asciidoc

Clarify that `preference` is a query string parameter only
and provide an example.
fubuki/elasticsearch,aparo/elasticsearch,aparo/elasticsearch,fubuki/elasticsearch,aparo/elasticsearch,aparo/elasticsearch,fubuki/elasticsearch,fubuki/elasticsearch,fubuki/elasticsearch,aparo/elasticsearch,aparo/elasticsearch,fubuki/elasticsearch