Clean up E0740 explanation

src/librustc_error_codes/error_codes/E0740.md

@@ -1,4 +1,4 @@
-A `union` cannot have fields with destructors.
+A `union` was declared with fields with destructors.
 
 Erroneous code example:
 
@@ -14,3 +14,5 @@ impl Drop for A {
     fn drop(&mut self) { println!("A"); }
 }
 ```
+
+A `union` cannot have fields with destructors.