Update wikipedia.py

I modified that line to avoid a error of encoding when asking for a section which has special characters. The error was produced in the format() function, so I removed it and concatenate the Strings. If I'm not wrong it does the same effect.

If you want to try to reproduce the error, get this article for example:

    #URL: http://es.wikipedia.org/wiki/Castillo_de_La_Mota
    wikipedia.set_lang("es")
    article = wikipedia.page('Castillo de la Mota')

Then, try to get the 3rd section, named 'Descripción':

    article.section(article.sections[2])

You will get the following error:

    UnicodeEncodeError: 'ascii' codec can't encode character u'\xf3' in position 9: ordinal not in range(128)