Correctly recurse print_tree method

When recursing the print_tree method, max_depth isn't passed in before the block, causing the block to be passed as max_depth which then results in an error trying to compare a fixnum to a block. This PR passes in all the arguments as expected.

There are currently no working tests for print_tree so I didn't modify or add any, let me know if there is more to be done for this to get merged. 

Thanks!