add booth in alias

Booth can be replaced with any alias by replacing it in alias.en.yml.

spec/controllers/booths_controller_spec.rb

@@ -55,7 +55,7 @@ describe BoothsController do
         end
 
         it 'shows success message' do
-          expect(flash[:notice]).to match('Booth successfully created.')
+          expect(flash[:notice]).to match("#{(t 'booth').capitalize} successfully created.")
         end
       end
 
@@ -74,7 +74,7 @@ describe BoothsController do
         end
 
         it 'shows flash message' do
-          expect(flash[:error]).to eq("Creating booth failed. Title can't be blank.")
+          expect(flash[:error]).to eq("Creating #{t 'booth'} failed. Title can't be blank.")
         end
       end
     end