fix source directory in person tutorial

[stuebinm]

in the package.yml snipped given in the Readme of the person tutorial,
the build directory was set to ".", which will fail with a Main.hs created
in the src directory (as described in the tutorial).

The actual code in the same directory appears to be fine.

(just noticed this while trying to follow the tutorial)

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[stuebinm]

Huh. I don't particularly feel like creating a google account and signing the agreement just for three new bytes in a readme, so i guess just take this PR as a bug report …