docs: fix reversed tuple order in multiprocessing example docstring

The 'Worker layout' docstring described tasks as `(expr, label)` but
the code builds and unpacks them as `(label, expr)`. Correct the doc
to match.

Author: timsaucer

examples/multiprocessing_pickle_expr.py

@@ -26,7 +26,7 @@
 
 Worker layout:
 
-* Each worker receives a different ``(expr, label)`` task.
+* Each worker receives a different ``(label, expr)`` task.
 * Each worker materializes the shared dataset locally and runs its
   own expression against it.
 * The result and the worker's PID travel back to the driver, so the