Remove mention of incorrect filename

If the `wiki.vim` configuration consists solely of `let g:wiki_root = '~/wiki'`, hitting `<leader>ww` actually opens `index.md` instead of `index.wiki`. This behavior changes when `let g:wiki_filetypes = [ 'wiki' ]` is added to the configuration. In the similar section in `:help wiki-intro-usage` no filename is mentioned, so I've updated `README.md` accordingly.