DOC fix estimator documentation in IHT

[glemaitre]

closes #889

Fix removing the information that one can pass some string. This is not supported since 5 years ago.

[codecov]

Codecov Report

Merging #890 (5fb5364) into master (e802a19) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #890   +/-   ##
=======================================
  Coverage   97.30%   97.30%           
=======================================
  Files          97       97           
  Lines        6337     6337           
  Branches      712      712           
=======================================
  Hits         6166     6166           
  Misses        102      102           
  Partials       69       69           

Impacted Files	Coverage Δ
...rototype_selection/_instance_hardness_threshold.py	100.00% <ø> (ø)

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