Update search.asciidoc
Update search.asciidoc

The constant `DOC_FIELD_NAME` used in the code snippet is in `SortParseElement` on 2.x branch.
strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test,strapdata/elassandra-test