Small but important correction

[BenSandeen]

In the section titled "Parallel Programming with numpy and scipy" in ParallelProgramming.ipynb, there's statement that while true in most cases, is not universally valid: "Using a multicore machine will provide at best a speedup by a factor of the number of cores available."

For things like searching a tree, depending on where the sought-after node is, it is possible to observe speedups of factors far greater than the number of cores available.

[Pauli Virtanen]

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