Hide FactoryBot deprecation warning by changing static assignment

Signed-off-by: Sebastian Serth <Sebastian.Serth@student.hpi.de>

spec/factories/code_ocean/file.rb

@@ -3,13 +3,13 @@ require 'seeds_helper'
 module CodeOcean
   FactoryBot.define do
     factory :file, class: CodeOcean::File do
-      content ''
+      content { '' }
       association :context, factory: :submission
       association :file_type, factory: :dot_rb
-      hidden false
+      hidden { false }
       name { SecureRandom.hex }
-      read_only false
-      role 'main_file'
+      read_only { false }
+      role { 'main_file' }
     end
   end
 end