Fix typos “a”→“an”

compiler/rustc_middle/src/traits/mod.rs

@@ -509,7 +509,7 @@ pub enum ImplSource<'tcx, N> {
     TraitUpcasting(ImplSourceTraitUpcastingData<'tcx, N>),
 
     /// ImplSource automatically generated for a closure. The `DefId` is the ID
-    /// of the closure expression. This is a `ImplSource::UserDefined` in spirit, but the
+    /// of the closure expression. This is an `ImplSource::UserDefined` in spirit, but the
     /// impl is generated by the compiler and does not appear in the source.
     Closure(ImplSourceClosureData<'tcx, N>),