Update discovery-ec2.asciidoc (#28925)
Update discovery-ec2.asciidoc (#28925)

* Be more clear about the need to defining tags in EC2 to be able to filter
ES instances.

strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,vroyer/elassandra