Docs style fixes: ``FOO`` vs `FOO` (reloaded)

Using single backticks and inline markup:
 
        :file:`app.py`, :file:`yourapp/templates`, :file:`/var/www/foo`
        :mimetype:`application/json`
        :command:`pip`, :option:`--debug`
        :exc:`TypeError`

Using double backticks and inline markup:

        ``True``, ``False`` and ``None``
        ``GET`` and ``POST``
        ``DEBUG``, ``SERVER_NAME``, ``PATH_INFO``
        ``/static``, ``/``
        ``Flask-Uploads``, ``X-Forwarded-Host``
        ``<body>``, ``|safe``
        ``with``, ``for``, ``self``
        ``.html``, ``.py``

Without backticks:

        `HTTP` -> HTTP

Fixed typo:

        requirements.rst -> requirements.txt