💅 wrap the tip from #15624 at 80 chars [ci skip]

guides/source/active_record_migrations.md

@@ -434,7 +434,8 @@ change_column_default :products, :approved, false
 This sets `:name` field on products to a `NOT NULL` column and the default
 value of the `:approved` field to false.
 
-TIP: Unlike `change_column` (and `change_column_default`), `change_column_null` is reversible.
+TIP: Unlike `change_column` (and `change_column_default`), `change_column_null`
+is reversible.
 
 ### Column Modifiers