fix test reference

Author: nikomatsakis

src/test/ui/lifetime-errors/ex3-both-anon-regions-both-are-structs-4.stderr

@@ -4,7 +4,7 @@ error[E0623]: lifetime mismatch
 15 | fn foo(mut x: Ref) {
    |               ---
    |               |
-   |               this type was declared with multiple lifetimes...
+   |               this type is declared with multiple lifetimes...
 16 |     x.a = x.b;
    |           ^^^ ...but data with one lifetime flows into the other here