fix(List): fix propTypes warning in ListItem

[layershifter]

Picked from #1224.

Fixes this warning:

Warning: Failed prop type: Invalid prop `column` of type `object` supplied to `Flex`, expected `boolean`.
    in Flex (created by ListItem)
    in ListItem (created by List)
    in ul (created by List)
    in List (created by ListExampleSelectable)

[codecov]

Codecov Report

Merging #1266 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1266   +/-   ##
======================================
  Coverage    72.1%   72.1%           
======================================
  Files         731     731           
  Lines        5592    5592           
  Branches     1612    1612           
======================================
  Hits         4032    4032           
  Misses       1555    1555           
  Partials        5       5

Impacted Files	Coverage Δ
packages/react/src/components/List/ListItem.tsx	100% <100%> (ø)	⬆️

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