Fix typos “a”→“an”

compiler/rustc_infer/src/traits/mod.rs

@@ -28,7 +28,7 @@ pub use self::project::{
 pub use rustc_middle::traits::*;
 
 /// An `Obligation` represents some trait reference (e.g., `i32: Eq`) for
-/// which the "impl_source" must be found. The process of finding a "impl_source" is
+/// which the "impl_source" must be found. The process of finding an "impl_source" is
 /// called "resolving" the `Obligation`. This process consists of
 /// either identifying an `impl` (e.g., `impl Eq for i32`) that
 /// satisfies the obligation, or else finding a bound that is in