Corrected imports in migrations docs

docs/migrations.rst

@@ -27,7 +27,7 @@ Here is a simple example of a ``migrations.py`` file which creates a table, then
 
 .. code-block:: python
 
-    from sqlite_utils import Database, Migrations
+    from sqlite_utils import Migrations
 
     migrations = Migrations("creatures")
 
@@ -51,6 +51,8 @@ Once you have a ``Migrations(name)`` collection with one or more migrations regi
 
 .. code-block:: python
 
+    from sqlite_utils import Database
+
     db = Database("creatures.db")
     migrations.apply(db)