docfix: name attribute for foreign key should be specified in index

[lowjoel]

Specifying

    create_table :course_assessment_question_multiple_response_options do |t|
      t.references :question,
                   null: false,
                   foreign_key: {
                     name: :fk__course_assessment_multiple_response_option_question,
                     references: :course_assessment_question_multiple_responses
                   }
    end

errors with a name too long error. This works:

    create_table :course_assessment_question_multiple_response_options do |t|
      t.references :question,
                   null: false,
                   index: {
                     name: :fk__course_assessment_multiple_response_option_question,
                   },
                   foreign_key: {
                     references: :course_assessment_question_multiple_responses
                   }
    end

[lowjoel]

(don't ask why my schema names are so long, the app is crazy complex...)