Fix typo in DebruijnIndex documentation

Co-authored-by: Rémy Rakic <lqd@users.noreply.github.com>

compiler/rustc_type_ir/src/lib.rs

@@ -119,7 +119,7 @@ pub struct TypeFlags: u32 {
     /// Bruijn index of 0, meaning "the innermost binder" (in this case, a
     /// fn). The region `'a` that appears in the second argument type (`&'a
     /// isize`) would then be assigned a De Bruijn index of 1, meaning "the
-    /// second-innermost binder". (These indices are written on the arrays
+    /// second-innermost binder". (These indices are written on the arrows
     /// in the diagram).
     ///
     /// What is interesting is that De Bruijn index attached to a particular