Formatting inside links not working

Formatting inside link descriptions does not parse correctly if the formatting is at the beginning or end of the link description.

## Example

```ruby
irb(main):017:0> Orgmode::Parser.new("[[a][~a~]]").to_html
=> "<p><a href=\"a\">~a~</a></p>\n"
```

The output should be `"<p><a href=\"a\"><code>a</code></a></p>\n"`.