Fixed docstring typo s/set-min-ns-level/set-ns-min-level/g

[alexpetrov]

I have just noticed a typo in docstring and coudn't pass by.

Take this opportunity to thank you, @ptaoussanis, for all your and other contributors hard work on Open Source!

[ptaoussanis]

@alexpetrov Hi Alexander!

Thanks a lot for the kind words, and for the fix - much appreciated! Have merged manually, will be included with next release 👍