Fix typos “a”→“an”

compiler/rustc_data_structures/src/tiny_list.rs

@@ -5,7 +5,7 @@
 //!
 //! - If you have a list that rarely stores more than one element, then this
 //!   data-structure can store the element without allocating and only uses as
-//!   much space as a `Option<(T, usize)>`. If T can double as the `Option`
+//!   much space as an `Option<(T, usize)>`. If T can double as the `Option`
 //!   discriminant, it will even only be as large as `T, usize`.
 //!
 //! If you expect to store more than 1 element in the common case, steer clear