[docs] Fix DateFnsAdapter import docs typo

[Domino987]

The Adapter comes from "@material-ui/pickers/adapter/date-fns"; instead of '@material-ui-pickers/adapter/date-fns';

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[cypress]

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Test summary

73 • 0 • 1 • 0

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Run details

Project	material-ui-pickers
Status	Passed
Commit	4945ca8
Started	Apr 22, 2020 1:27 PM
Ended	Apr 22, 2020 1:29 PM
Duration	01:26 💡
OS	Linux Debian - 9.11
Browser	Chrome 78

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[codecov]

Codecov Report

Merging #1699 into next will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             next    #1699   +/-   ##
=======================================
  Coverage   89.27%   89.27%           
=======================================
  Files          73       73           
  Lines        2322     2322           
  Branches      405      423   +18     
=======================================
  Hits         2073     2073           
  Misses        198      198           
  Partials       51       51           

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