Fix typo in fn variance

[aidanhs]

Per #738 (comment)

I'm a little unsure though since this hasn't been fixed for so long?

[eddyb]

cc @rust-lang/lang

[aidanhs]

Finally, there can be cases where it is ok to make a lifetime longer, but not shorter. This comes up when the lifetime is used in a function return type (and only a fn return type).

By the same line of thought, I think this is wrong as well and should refer to function arguments?

I.e. it is safe to treat a fn(&'a T) as a fn(&'static T)

[nikomatsakis]

Looks like a correct typo.

[nikomatsakis]

Er, a correct fix =)