Fix grammar in extension description

[waldyrious]

I would also recommend changing the <strong> headings to use an actual heading tag, say <h3> or below. Was it intentional not to use them?

[maxlath]

For the headings, I can't exactly remember, it was 5 years ago and not documented ^^ It might be appearing too big then, but lets see, I did the change in 62e735f