[taskdistribution] Fix documentation

The module is to access Distributor not the whole tool.

erp5/util/README.taskdistribution.txt

 erp5.util.taskdistribution
 --------------------------
 
-Module to access TaskDistributionTool, used to run test on several machines
+Module to access TaskDistributor, used to run test on several machines
 and aggregating results.
 Use pydoc to get module documentation and usage example.