[documentation] typo in ReST markup

[Takuya Futatsugi]

This is documentation bug report.

## As is

https://www.crummy.com/software/BeautifulSoup/bs4/doc/_sources/index.rst.txt
In "Multi-valued attributes" section, I found a typo:

>You can use ```get_attribute_list`` to get a value that's always a

It causes a redundant "`" in the document:
https://www.crummy.com/software/BeautifulSoup/bs4/doc/#multi-valued-attributes
>You can use `get_attribute_list to get a value that’s always a

## To be

You can use ``get_attribute_list`` to get a value that's always a

[Leonard Richardson (leonardr)]

Fixed in revision 603.