Typo in docs for qmutilde test-statistic

[andrewfowlie]

Minor typo, though worth fixing

https://github.com/scikit-hep/pyhf/blob/5b632519838f6ad2c73a8c8f8b53a09c1d8c678b/src/pyhf/infer/test_statistics.py#L174-L180

On L176, I think a \hat{\mu} should be replaced by 0. This is in the denominator of the \hat{\mu} < 0 case. It's easier to read in the latex rendered docs

https://scikit-hep.org/pyhf/_generated/pyhf.infer.test_statistics.qmu_tilde.html#pyhf.infer.test_statistics.qmu_tilde