Add dyn keyword to E0373 examples

compiler/rustc_error_codes/src/error_codes/E0373.md

@@ -3,7 +3,7 @@ A captured variable in a closure may not live long enough.
 Erroneous code example:
 
 ```compile_fail,E0373
-fn foo() -> Box<Fn(u32) -> u32> {
+fn foo() -> Box<dyn Fn(u32) -> u32> {
     let x = 0u32;
     Box::new(|y| x + y)
 }
@@ -42,7 +42,7 @@ This approach moves (or copies, where possible) data into the closure, rather
 than taking references to it. For example:
 
 ```
-fn foo() -> Box<Fn(u32) -> u32> {
+fn foo() -> Box<dyn Fn(u32) -> u32> {
     let x = 0u32;
     Box::new(move |y| x + y)
 }