fix alembic doc format

Author: wwwjfy

docs/how-to/alembic.rst

@@ -29,11 +29,11 @@ Inside ``models.py`` define simple DB Model with GINO:
 
     db = Gino()
 
-        class User(db.Model):
-            __tablename__ = 'users'
+    class User(db.Model):
+        __tablename__ = 'users'
 
-     id = db.Column(db.Integer(), primary_key=True)
-     nickname = db.Column(db.Unicode(), default='noname')
+        id = db.Column(db.Integer(), primary_key=True)
+        nickname = db.Column(db.Unicode(), default='noname')
 
 
 Set up Alembic