Update mix phx.digest part

[konstantinzolotarev]

Summary of changes

Updated cd command because now if PHOENIX_SUBDIR need to be changed to something like apps/web_api it will fail on this step.

Checklist

• [x] New functions have typespecs, changed functions were updated
• [x] Same for documentation, including moduledocs
• [x] Tests were added or updated to cover changes
• [x] Commits were squashed into a single coherent commit

[bitwalker]

Thanks!