Max block does not work in GRC when IO Type is Short

Because of a typo in blocks_max_xx.xml, the Max block does not work in GNU Radio Companion when the IO Type is set to Short.

To reproduce the problem, create a flow graph with a null source connected to a Max block connected to a null sink, and set all three blocks to use Short.  Click the execute button, and you'll see the following error:

Traceback (most recent call last):
  File "/home/argilo/Documents/gnuradio/top_block.py", line 62, in <module>
    tb = top_block()
  File "/home/argilo/Documents/gnuradio/top_block.py", line 34, in __init__
    self.blocks_max_xx_0 = blocks.max_dd(1)
AttributeError: 'module' object has no attribute 'max_dd'

After applying this patch, the flow graph executes correctly.