Hypotests docs: error with non-negative scale when generating random points

[amanmdesai]

At present : bkg = np.random.exponential(-1/2, 300) uses a non-negative scale which results in error.

Solution:
According to the numpy docs, the scale must be non-negative. Link

[amanmdesai]

I have included the changes in #71

[amanmdesai]

Done