Update docs to disable rule in DB guides example

In the example for "Creating a new table when we have two foreign keys"
we recommend using `add_foreign_key` but we have a Rubocop rule that
warns against its usage - Migration/AddConcurrentForeignKey.

This updates the examples too disable this rule.

doc/development/migration_style_guide.md

@@ -300,7 +300,7 @@ include Gitlab::Database::MigrationHelpers
 
 def up
   with_lock_retries do
-    add_foreign_key :imports, :projects, column: :project_id, on_delete: :cascade
+    add_foreign_key :imports, :projects, column: :project_id, on_delete: :cascade # rubocop:disable Migration/AddConcurrentForeignKey
   end
 end
 
@@ -318,7 +318,7 @@ include Gitlab::Database::MigrationHelpers
 
 def up
   with_lock_retries do
-    add_foreign_key :imports, :users, column: :user_id, on_delete: :cascade
+    add_foreign_key :imports, :users, column: :user_id, on_delete: :cascade # rubocop:disable Migration/AddConcurrentForeignKey
   end
 end