fix Class doc: update Greeter to greeter

[bmeritter]

Fixes #

[orta]

I think (though it does feel ambiguous) that this sentence refers to the first line in the code sample which means this fix is probably wrong. So I'll close this - thanks for giving an improvement a shot! It means a lot when folks help improve the core docs :D

[bmeritter]

@orta I just think Greeter is a Class, but greeter is an object, greeter is an instance of Greeter. We should let it clear.

[orta]

Sure if you want to rename the instance in the docs that could help?

[bmeritter]

@orta Yeah