Migrations: Fix opening bullets.

The wording of these was a bit off, so I fixed them.

guides/source/migrations.md

@@ -10,8 +10,8 @@ In this guide, you'll learn all about migrations including:
 
 * The generators you can use to create them
 * The methods Active Record provides to manipulate your database
-* The Rake tasks that manipulate them
-* How they relate to `schema.rb`
+* The Rake tasks that manipulate migrations and your schema
+* How migrations relate to `schema.rb`
 
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