Merge branch 'migration_guide_remove_rubocop' into 'master'

Remove Rubocop skip comment from migration style guide

This was an error in previous MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4693

[ci skip]

See merge request !4879

doc/development/migration_style_guide.md

@@ -37,7 +37,6 @@ First, you need to provide information on whether the migration can be applied:
 For example: 
 
 ```
-# rubocop:disable all
 # Migration type: online without errors (works on previous version and new one)
 class MyMigration < ActiveRecord::Migration
 ...