DOC fixes some documentation glitches

[glemaitre]

Fix a couple of documentation glitches

[codecov]

Codecov Report

Merging #876 (d8f6c75) into master (67d2122) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #876      +/-   ##
==========================================
- Coverage   97.23%   97.18%   -0.05%     
==========================================
  Files          96       96              
  Lines        6253     6253              
  Branches      712      712              
==========================================
- Hits         6080     6077       -3     
- Misses        102      104       +2     
- Partials       71       72       +1     

Impacted Files	Coverage Δ
imblearn/datasets/tests/test_zenodo.py	91.89% <0.00%> (-2.71%)	⬇️
imblearn/keras/_generator.py	92.10% <0.00%> (-2.64%)	⬇️

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