Fix build script and README

[mirgj]

Description of changes:

The build-s3-dist.sh script was expecting 4 input parameters while only 3 were used inside the script (the second parameter was never used in the script).

The doc was referring to ./dist folder that is not generated by the script and I guess the correct value is ./regional-s3-assets (where all the code, lambda are placed by the script)

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[f00f]

lgtm