[Docs] Feature selection states wrong number of best features.

[dor132]

What does this implement/fix? Explain your changes
It seems that the grid CV concluded that 5 features provided the best model, but the text after that output stated 3 instead of 5.
Or, Celltype is considered one feature?
In this case, maybe emphasizing this will make things clearer for new learners.

This is a simple one digit change in the docs.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (942efd3) 97.85% compared to head (2b3a1c9) 97.85%.

Additional details and impacted files

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##           master     #345   +/-   ##
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  Coverage   97.85%   97.85%           
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  Files          37       37           
  Lines        3270     3270           
  Branches      496      496           
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  Hits         3200     3200           
  Misses         33       33           
  Partials       37       37           

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