Fix small typo in one exemple of the User and Programming 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 "example.py", line 12, in <module>
    print worker.node_buffer(node)
NameError: name 'worker' is not defined
```
Replacing `worker` by `task` obviously fix the issue.