There's no apostrophe in 90s

Apostrophes are for indicating an abbreviation or for denoting possession. '90s' is merely a plural. It is an abbreviation of 1990s, so technically it ought to be '90s, but for all intents and purposes this is optional.
Updated note regarding Python 3 tutorial.
Merge pull request #855 from jackhooper/master

Got rid of misplaced apostrophe & updated note on Python 3 tutorial.