Fix missing translation

app/views/activities/edit.html.haml

 %h1
-  = t 'activity.editing'
+  = t 'activities.edit'
 
 = render 'form', activity: @activity
 

config/locales/activities/en.yml

 
     new: "New activity"
     created: "Activity created."
+    edit: "Edit activity"
     updated: "Activity updated."
     destroyed: "Activity destroyed."
     mass_import: "Create multiple activities in %{group}"

config/locales/activities/nl.yml

 
     new: "Nieuwe activiteit"
     created: "Activiteit aangemaakt."
+    edit: "Activiteit bewerken"
     updated: "Activiteit bijgewerkt."
     destroyed: "Activiteit verwijderd."
     mass_import: "Meerdere activiteiten aanmaken in %{group}"