Fixed typo in forms API documentation.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15896 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/ref/forms/api.txt

@@ -384,7 +384,7 @@ errors. For example, you might want to present required form rows in bold and
 highlight errors in red.
 
 The :class:`Form` class has a couple of hooks you can use to add ``class``
-attributes to required rows or to rows with errors: simple set the
+attributes to required rows or to rows with errors: simply set the
 :attr:`Form.error_css_class` and/or :attr:`Form.required_css_class`
 attributes::