Update comment in contravariant test

The previous definition was actually describing covariance.
Fixing to describe contravariance while keeping 'static in the definition was tricky so just changed to use 'short and 'long.

Author: jakerr

src/test/compile-fail/regions-variance-contravariant-use-covariant.rs

@@ -15,8 +15,8 @@
 // variance inference works in the first place.
 
 // This is contravariant with respect to 'a, meaning that
-// Contravariant<'foo> <: Contravariant<'static> because
-// 'foo <= 'static
+// Contravariant<'long> <: Contravariant<'short> iff
+// 'short <= 'long
 struct Contravariant<'a> {
     f: &'a int
 }