[naga] Fix type error in test.

Change the WGSL code in the `function_returns_void` test in
`tests/wgsl-errors.rs` so that the construction expression types
correctly. Subsequent iterations of the WGSL front end will be doing
some type checking earlier, to support WGSL's automatic conversions,
and without this fix, this test will stop checking what it is intended
to check.

naga/tests/wgsl-errors.rs

@@ -1909,7 +1909,7 @@ fn function_returns_void() {
     check(
         "
         fn x() {
-	        let a = vec2<f32>(1, 2u);
+	        let a = vec2<f32>(1.0, 2.0);
         }
 
         fn b() {