docs: adds the build arg to Builder in CDK docs

[kirkness]

Some background in @ibrahimcesar's comment here.

When running the builder with the defaults, e.g. new Builder('.', './build').build() the process will hang as the builder just runs next with no args, which in-turn starts the next dev server. Not sure whether this is intended or something has changed as I don't recall this behavior - high chance I've just forgotten. Either way, this PR just updates the docs to avoid confusion.

[codecov]

Codecov Report

Merging #908 (7fce605) into master (83c8c6d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #908   +/-   ##
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  Coverage   81.31%   81.31%           
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  Files          68       68           
  Lines        2488     2488           
  Branches      591      591           
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  Hits         2023     2023           
  Misses        398      398           
  Partials       67       67           

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