Add missing alias to PredictiveEntropy

[MiWeiss]

The docs (https://uncertainty-wizard.readthedocs.io/en/latest/user_guide_quantifiers.html) mention "PE" as alias for predictive entropy, but it was not actually implemented. I now added the alias.

[codecov]

Codecov Report

Merging #86 (315f94b) into main (5ba9bfc) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #86   +/-   ##
=======================================
  Coverage   94.67%   94.67%           
=======================================
  Files          27       27           
  Lines         901      901           
=======================================
  Hits          853      853           
  Misses         48       48           

Impacted Files	Coverage Δ
...certainty_wizard/quantifiers/predictive_entropy.py	100.00% <100.00%> (ø)

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