[docs] punctuation errors at https://docs.python.org/3/tutorial/classes.html#generators

[keith.briggs at bt.com]

There are a couple of punctuation errors at https://docs.python.org/3/tutorial/classes.html#generators:

1. "left-off" should be "left off" (it's pronounced as two words, so should not be hyphenated)

2. "class based iterators" should be "class-based iterators".

Keith