Fix small typo in one exemple of User Guide

There was a small typo in the sequential remote command example of the User and Programming Guide.

Blind running the given example raised the following exception:
Traceback (most recent call last):
  File "apply-puppet.py", line 12, in <module>
    print worker.node_buffer(node)
NameError: name 'worker' is not defined

Replacing worker by task obviously fix the issue.