arrow

MINOR: [Ruby][Docs] Add a short example of Arrow::Table#join to README (#33922)

### Rationale for this change

The code for `Apache::Table` `join` has recently been merged (#12108, #30086) into the Ruby gem, but the current documentation for `join` suggests referring to Jira, which then links into GitHub, where the commit can be found.

This change instead provides a short example of a join.

### What changes are included in this PR?

I have added an example to the README and removed the link to the now-completed bug report.

### Are these changes tested?

No, but is documentation only.

### Are there any user-facing changes?

Yes - this PR is documentation.

Authored-by: Robin Duncan Sharp <rob@sharp.id.au>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>