Adjusted documentation of tableList command (Java)

[clovergaze]

• provided the correct return type
• extended the example

Reason for the change
I tried to simply get the list of table names in a database and the current documentation is not helpful in that regard. The specified return type is wrong and the array with table names had to be extracted from the Result<Object>.

Description
I corrected the return type and added a useful example that shows how to really get the table names.

Checklist

• I have read and agreed to the RethinkDB Contributor License Agreement

[clovergaze]

I made a Git related mistake and had to submit this again here #1288.