docs(MenuExampleHeaderVertical): fix onClick handler

[Suzi004]

handleItemClick = (e, { name }) => this.setState({ activeItem: name });

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[codecov-io]

Codecov Report

Merging #3799 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3799   +/-   ##
=======================================
  Coverage   99.83%   99.83%           
=======================================
  Files         172      172           
  Lines        3101     3101           
=======================================
  Hits         3096     3096           
  Misses          5        5

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