copy editing [ci skip]

guides/source/migrations.textile

@@ -51,7 +51,7 @@ end
 
 This migration adds a table called +products+ with a string column called +name+
 and a text column called +description+. A primary key column called +id+ will
-also be added, however since this is the default we do not need to explicitly write for this.
+also be added, however since this is the default we do not need to explicitly specify it.
 The timestamp columns +created_at+ and +updated_at+ which Active Record
 populates automatically will also be added. Reversing this migration is as
 simple as dropping the table.
@@ -65,7 +65,7 @@ class AddReceiveNewsletterToUsers < ActiveRecord::Migration
     change_table :users do |t|
       t.boolean :receive_newsletter, :default => false
     end
-    User.update_all :receive_newsletter =>  true
+    User.update_all :receive_newsletter => true
   end
 
   def down
@@ -636,9 +636,8 @@ $ rake db:migrate:up VERSION=20080906120000
 </shell>
 
 will run the +up+ method from the 20080906120000 migration. This task will first
-check whether the migration is already performed, so for example
- +db:migrate:up VERSION=20080906120000+ will do nothing if Active Record believes
-  that 20080906120000 has already been run.
+check whether the migration is already performed and will do nothing if Active Record believes
+that it has already been run.
 
 h4. Changing the output of running migrations