Fix docs describing rollback [ci skip]

* `rails db:migrate STEP = 2` will not rollback the migrations, instead
   `rails db:rollback STEP = 2` will do the rollback.
* Also, rewritten `rails db:migrate VERSION` => `rails db:rollback VERSION`
  for consistency.

activerecord/lib/active_record/migration.rb

@@ -354,9 +354,9 @@ def initialize(current: nil, stored: nil)
   # to match the structure of your database.
   #
   # To roll the database back to a previous migration version, use
-  # <tt>rails db:migrate VERSION=X</tt> where <tt>X</tt> is the version to which
+  # <tt>rails db:rollback VERSION=X</tt> where <tt>X</tt> is the version to which
   # you wish to downgrade. Alternatively, you can also use the STEP option if you
-  # wish to rollback last few migrations. <tt>rails db:migrate STEP=2</tt> will rollback
+  # wish to rollback last few migrations. <tt>rails db:rollback STEP=2</tt> will rollback
   # the latest two migrations.
   #
   # If any of the migrations throw an <tt>ActiveRecord::IrreversibleMigration</tt> exception,