Highlight the ShEx code with ShEx

At this moment the ShEx code is highlighted using the Turtle parser because the bikeshed seems to complain when specifying ShEx or ShExC even if those parsers are supported by [Pygments lexers](https://pygments.org/docs/lexers/).

The line is [this one](https://github.com/Arquisoft/viadeSpec/blob/master/main/introduction.bs#L41).

Start at main/introduction.bs line 41, where the ShEx code is currently assigned Turtle highlighting. Check how Bikeshed handles the ShEx and ShExC lexer names supported by Pygments, then build the specification to verify it accepts the chosen lexer. Done means the ShEx code is highlighted as ShEx rather than Turtle.

Written by the indexing model from the issue text.