[1.2.X] Changed ModelForm.fields and ModelForm.exclude examples to use tuples...

[1.2.X] Changed ModelForm.fields and ModelForm.exclude examples to use tuples instead of lists since they were used inconsistently throughout the page (it wasn't hurting anything, but consistency is nice). Thanks to lspcity for the report and gruszczy for the patch.

Backport of [14134] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14135 bcc190cf-cafb-0310-a4f2-bffc1f526a37