Fixed tag url error in sidebar(gum theme).

In gum theme.
if post tag is Chinese. The pelican generated file name for tag is pinyin for Chinese.

```
In pelican:
[ "你好" tag ] convert to [ ni-hao.html ] 
```

So, used ```{{ tag.0 }}``` in tag link is not idea. Because ```{{ tag.0 }}``` is "你好" not "ni-hao".
May be the problem also exist in other language. I humbly recommend ```{{ tag.0.url }}```. It will generate "ni-hao.html".

thx.
:P