spelling: contravariant

Signed-off-by: Josh Soref <2119212+jsoref@users.noreply.github.com>

tests/ui/variance/variance-trait-object-bound.rs

@@ -1,6 +1,6 @@
 // Checks that regions which appear in a trait object type are
 // observed by the variance inference algorithm (and hence
-// `TOption` is contavariant w/r/t `'a` and not bivariant).
+// `TOption` is contravariant w/r/t `'a` and not bivariant).
 //
 // Issue #18262.