[MRG] Consistent spelling error in documentation

[gnsiva]

Just fixing the spelling of "multi".

Issue: #430

[codecov]

Codecov Report

Merging #432 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #432   +/-   ##
=======================================
  Coverage   98.71%   98.71%           
=======================================
  Files          70       70           
  Lines        4188     4188           
=======================================
  Hits         4134     4134           
  Misses         54       54

Impacted Files	Coverage Δ
...ampling/prototype_selection/one_sided_selection.py	100% <ø> (ø)	⬆️
...g/prototype_selection/edited_nearest_neighbours.py	100% <ø> (ø)	⬆️
imblearn/combine/smote_enn.py	100% <ø> (ø)	⬆️
...mpling/prototype_selection/random_under_sampler.py	100% <ø> (ø)	⬆️
imblearn/ensemble/easy_ensemble.py	100% <ø> (ø)	⬆️
...arn/under_sampling/prototype_selection/nearmiss.py	98.68% <ø> (ø)	⬆️
...sampling/prototype_generation/cluster_centroids.py	100% <ø> (ø)	⬆️
imblearn/over_sampling/adasyn.py	98.57% <ø> (ø)	⬆️
...prototype_selection/neighbourhood_cleaning_rule.py	100% <ø> (ø)	⬆️
.../under_sampling/prototype_selection/tomek_links.py	100% <ø> (ø)	⬆️
... and 6 more

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[glemaitre]

Thanks !!!