Fix attribute signature html formatting

[FroyaTheHen]

The problem:
let's take

            #[doc(group: "this is visible only within the scope")]
            struct Six {}

the signature in markdown will be evaluated to:

<pre><code class="language-cairo">#[doc(group: &quot;test group 3&quot;)]
struct Three {}
</code></pre>

then the generated page will look like:

even though the attribute #[doc(group: "this is visible only within the scope")] is present it cannot be seen.

Solution:
It should either be removed from the signature or made visible (by for example including an extra <span> around the signature - not sure though if that's the best solution).