codelabs-android-barcode_detector: weird description in step 06

[marlonlom]

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When reading the step 06 in the following codelab, it mentions faces where the exercise is about scanning barcodes.

Codelab: Barcode Detection with the Mobile Vision API URL: https://codelabs.developers.google.com/codelabs/bar-codes/#5 Evidence: "This application has a single button that will load the image, detect any faces on it, and draw a red rectangle around them when it does. Let's write the code to achieve this:"

Fix: faces => barcodes