use YAML.load_file in database tasks example

rather than YAML.load(File.read(path)). YAML.load_file is also used in
guides/rails_guides/helper.rb since 2011, the only other precedent I could find.

activerecord/lib/active_record/tasks/database_tasks.rb

@@ -28,7 +28,7 @@ class DatabaseNotSupported < StandardError; end # :nodoc:
     # Example usage of +DatabaseTasks+ outside Rails could look as such:
     #
     #   include ActiveRecord::Tasks
-    #   DatabaseTasks.database_configuration = YAML.load(File.read('my_database_config.yml'))
+    #   DatabaseTasks.database_configuration = YAML.load_file('my_database_config.yml')
     #   DatabaseTasks.db_dir = 'db'
     #   # other settings...
     #