Fixed #23169 - documentation small fix

related to ticket https://code.djangoproject.com/ticket/23169
In the title using ``field`` is not very elegant: https://docs.djangoproject.com/en/dev/ref/forms/fields/#built-in-field-classes
and it get worst in the top sidebar: https://docs.djangoproject.com/en/dev/ref/forms/fields/

Looking at Sphinx doc: http://sphinx-doc.org/rest.html, it's an inline markup 
backquotes: ``text`` for code samples.