Mixed query strategies have misleading parameter name `mix_ratio`

[Jurriaan H. Spaaks]

The mixed query strategies MaxRandomQuery, MaxUncertaintyQuery and MixedQuery use a confusingly named parameter mix_ratio whose name suggests that the ratio is between the two strategies, which it is not.

Relevant file:

https://github.com/asreview/asreview/blob/caf788b36aa1483556e903448b20aa53ba0c4995/asreview/models/query/mixed.py