Fix mixed use of "a" / "an" article in E0277

[ElectrifyPro]

This PR fixes a tiny little annoyance typo in error E0277. Currently, this code fails to compile (correctly) with the following error:

struct A<F: Fn()>(F);

impl<F> A<F> {}

fn main() {}

error[E0277]: expected a `Fn()` closure, found `F`
 --> src/main.rs:3:9
  |
3 | impl<F> A<F> {}
  |         ^^^^ expected an `Fn()` closure, found `F`
  |
  = note: wrap the `F` in a closure with no arguments: `|| { /* code */ }`
... truncated

Unfortunately, the first line of the error message uses the article "a", when it should be "an" for the Fn trait and friends. The help label does use the correct article, "an".

Unless if I have been pronouncing Fn incorrectly...

[rustbot]

r? @petrochenkov

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