Fix typo "recipeint" in metamta.one-mail-per-recipient description

Test Plan: Go to /config/settings/all and look at the description of metamta.one-mail-per-recipient

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25311