Use actual method name

[Andre601]

Reason for the change The documentation for the java driver mentions default at the top of the page (i.e. value.default(default_value) -> any) while the method itself is called default_ due to java having a primitive called default.

Description This PR modifies the java doc by actually showing the right method to prevent possible confusion.

Code examples

Checklist

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

References Changes made apply to the following docs https://rethinkdb.com/api/java/default/

[srh]

Thanks.