Update README.rst to correct the number of re-sampling techniques

[gaugup]

In README.rst it mentions that there are two techniques instead of four.

What does this implement/fix? Explain your changes.

Any other comments?

[codecov]

Codecov Report

Merging #909 (391687a) into master (6176807) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #909      +/-   ##
==========================================
+ Coverage   97.19%   97.22%   +0.03%     
==========================================
  Files          97       97              
  Lines        6348     6348              
  Branches      711      711              
==========================================
+ Hits         6170     6172       +2     
+ Misses        106      105       -1     
+ Partials       72       71       -1     

Impacted Files	Coverage Δ
imblearn/keras/_generator.py	94.73% <0.00%> (+2.63%)	⬆️

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[glemaitre]

Thanks @gaugup