Fix grammar in www/docs.html

edits page “Getting Started: Quick Overview”, at http://gosu-lang.org/docs.html

“However” must start a sentence; it can’t be in the middle of a sentence like “but” can. Using a semicolon effectively starts a new sentence.

Removed “It’s true that” because, as a newbie, I never had the impression that Gosu was strongly typed – I had no expectations. But “It’s true that” sounds like a response to an expectation.