Small bug fix - completed by email now shows

Wrong method being used to get supplier email for completion.

Fixes: https://www.agileplannerapp.com/boards/187120/cards/7978

Although I didn't quit understand the note on the card "... It should not change when re-saving a service that is already completed." - I assume this means that clicking the save button on the summary page shouldn't change the "completed by...." section, which it doesn't.

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