No need to install OpenJDK for ElasticSearch

[danielcolquitt]

First time contributor so please forgive any faux pas.

ElasticSearch comes bundled with its own JVM and its neither necessary nor desirable to install a standalone JVM as currently suggested in the documentation. I've opened a pull request (https://github.com/mastodon/documentation/pull/1597) to modify the relevant part of the documentation, but I wasn't sure whether I also needed to open an issue.

I've just finished setting up ES on my own instance and think the docs could benefit from some additions/modifications. I'd be happy to do this once I know how the community prefer to receive updates to the docs (issues + pull requests vs. just pull requests).