remove `<code class="hljs">`

[VladimirAlexiev]

Turtle code examples use black text:

JSONLD uses gray text:

SHACLC also uses gray text:

Why gray not black? Black has better contrast so is more readable.

I believe this comes from <code class="hljs"> that is nested in <pre>:

As an alternative to implementing useful code highlighting (#465), please remove this element