Merge pull request #923 from fasouto/master

Docs: Missing single quotes in form_extra_fields example

flask_admin/model/base.py

@@ -444,7 +444,7 @@ class BaseModelView(BaseView, ActionsMixin):
 
             class MyModelView(BaseModelView):
                 form_extra_fields = {
-                    password: PasswordField('Password')
+                    'password': PasswordField('Password')
                 }
 
         You can control order of form fields using ``form_columns`` property. For example::
@@ -453,7 +453,7 @@ class BaseModelView(BaseView, ActionsMixin):
                 form_columns = ('name', 'email', 'password', 'secret')
 
                 form_extra_fields = {
-                    password: PasswordField('Password')
+                    'password': PasswordField('Password')
                 }
 
         In this case, password field will be put between email and secret fields that are autogenerated.