fix(FlexItem): fix children type

[mnajdova]

FIxing this console warning:

react.development.js:2726 Warning: Failed prop type: Invalid prop `children` supplied to `FlexItem`, expected a ReactNode.
    in FlexItem (created by ComponentExampleKnobs)
    in ComponentExampleKnobs (created by KnobInspector)
    in KnobInspector (created by ComponentExample)
    in div (created by Segment)
    in Segment (created by ComponentExample)
    in KnobProvider (created by ComponentExample)
    in FlexItem (created by ComponentExample)
    in div (created by Flex)
    in Flex (created by ComponentExample)
    in ComponentExample (created by ComponentSourceManager)
   ...

[codecov]

Codecov Report

Merging #1320 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1320   +/-   ##
=======================================
  Coverage   72.32%   72.32%           
=======================================
  Files         759      759           
  Lines        5692     5692           
  Branches     1687     1663   -24     
=======================================
  Hits         4117     4117           
  Misses       1569     1569           
  Partials        6        6

Impacted Files	Coverage Δ
packages/react/src/components/Flex/FlexItem.tsx	78.57% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bca7430...62eeea4. Read the comment docs.

[miroslavstastny]

nit: children argument in createCommon can be boolean or string. Would it make sense to pass direct value (like you do on line 57) as well (or instead of the string)?

[mnajdova]

That's an interesting question. I can add as a separate PR options for passing direct value for the children and content options.

[codecov]

Codecov Report

Merging #1320 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1320   +/-   ##
=======================================
  Coverage   72.33%   72.33%           
=======================================
  Files         759      759           
  Lines        5694     5694           
  Branches     1664     1688   +24     
=======================================
  Hits         4119     4119           
  Misses       1569     1569           
  Partials        6        6

Impacted Files	Coverage Δ
packages/react/src/components/Flex/FlexItem.tsx	78.57% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 13da159...96430b4. Read the comment docs.