Fix typo in number of tasks in Splitters.rst

[dg1223]

In the documentation page for Splitters, it is mentioned that the prime factorisation job for a very large number is being split into 5 different tasks whereas the code for job declation actually splits it up into 10 subjobs:

We can split up the factorisation of a very large number up into 5 different tasks.
splitter = PrimeFactorizerSplitter(numsubjobs=10))

This PR fixes the typo to resolve the discrepancy in the documentation.

[dg1223]

Just wanted to add that I ran tests (python -m pytest ganga/GangaCore/test/Unit) after making the change and they all passed as expected since this is just a documentation fix.

[egede]

Thanks for the fix.