DOC fix the documentation to use substitution

[glemaitre]

Use string substitution in docstring of SMOTENC

[pep8speaks]

Hello @glemaitre! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file imblearn/over_sampling/_smote.py:

Line 787:89: E501 line too long (91 > 88 characters)

[codecov]

Codecov Report

Merging #805 (ab03319) into master (326119b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #805   +/-   ##
=======================================
  Coverage   98.63%   98.63%           
=======================================
  Files          89       89           
  Lines        5933     5934    +1     
  Branches      498      499    +1     
=======================================
+ Hits         5852     5853    +1     
  Misses         80       80           
  Partials        1        1           

Impacted Files	Coverage Δ
imblearn/over_sampling/_smote.py	97.31% <100.00%> (+<0.01%)	⬆️

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