Documentation: fix sample code in chapter "Validation"

[deining]

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[codecov-commenter]

Codecov Report

Merging #1155 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1155      +/-   ##
============================================
+ Coverage     94.25%   94.30%   +0.04%     
  Complexity      455      455              
============================================
  Files             2        2              
  Lines          6667     6667              
  Branches       1791     1792       +1     
============================================
+ Hits           6284     6287       +3     
+ Misses          105      102       -3     
  Partials        278      278              

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/picocli/CommandLine.java	94.14% <0.00%> (+0.04%)	314.00% <0.00%> (ø%)

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[remkop]

Thanks for the review and the fixes!
I should obviously run the sample code before putting it in the docs! 😳

Merged and published.
Thanks again!