correct wording in hir docs

Author: Erk-

compiler/rustc_hir/src/hir.rs

@@ -1884,7 +1884,7 @@ pub enum PatKind<'hir> {
     Binding(BindingMode, HirId, Ident, Option<&'hir Pat<'hir>>),
 
     /// A struct or struct variant pattern (e.g., `Variant {x, y, ..}`).
-    /// The `Option` contains the span a possible `..`.
+    /// The `Option` contains the span of a possible `..`.
     Struct(QPath<'hir>, &'hir [PatField<'hir>], Option<Span>),
 
     /// A tuple struct/variant pattern `Variant(x, y, .., z)`.