Update plugin building documentation wrt jar-dependencies gemfile use

Currently in the documentation the example of using jar-dependencies in the Gemfile marks this gem as a runtime dependency.

This has the inconvenience of making jar-dependencies fetch jars at gem install time, instead of build time.

The references to jar-dependency shoud then be changed from:

`s.add_runtime_dependency 'jar-dependencies'``
to
`s.add_development_dependency 'jar-dependencies'`

Example of occurrence: https://www.elastic.co/guide/en/logstash/current/_how_to_write_a_logstash_filter_plugin.html#_jar_dependencies_3