grammar fix (#2976)

Note it is a very "visible" grammar error for the French user, and it is in the confirmation mail after team creation. Though it may seem a minor issue we wish it can be corrected asap because we just started our "Framateam" project based on mattermost https://framateam.org/ - TIA

i18n/fr.json

@@ -1229,7 +1229,7 @@
   },
   {
     "id": "api.templates.signup_team_body.title",
-    "translation": "Merci d'avoir créer une équipe !"
+    "translation": "Merci d'avoir créé une équipe !"
   },
   {
     "id": "api.templates.signup_team_subject",