web/introduction.itexi: replace opera series with opera seria

[Federico Bruni]

Can anybody fix this typo in the website?
http://lilypond.org/productions.html

/opera series/opera seria/

See https://en.wikipedia.org/wiki/Opera_seria

It was reported by Luca Rossetto.
He suggests the following wording:

Luca Rossetto Casel, for his Ph.D. thesis, created a critical edition 
of Tommaso Traetta’s Enea nel Lazio (1760), opera seria on a libretto 
by Vittorio Amedeo Cigna-Santi, in four parts:
...

I don't have time to go through the patch process.

Thanks
Federico