Fix error while rendering links with absolute URI inside of <sup> tag

Without this fix it raised the following error:

```
     Nokogiri::CSS::SyntaxError:
       unexpected '//' after ':'
     # ./lib/qiita/markdown/filters/footnote.rb:8:in `block in call'
     # ./lib/qiita/markdown/filters/footnote.rb:6:in `call'
     # ./lib/qiita/markdown/processor.rb:37:in `call'
```