Fix code examples

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0752.md

@@ -3,8 +3,8 @@ The entry point of the program was marked as `async`.
 Erroneous code example:
 
 ```compile_fail,E0752
-async fn main() -> Result<i32, ()> { // error!
-    Ok(1)
+async fn main() -> Result<(), ()> { // error!
+    Ok(())
 }
 ```
 
@@ -13,7 +13,7 @@ having a correct async runtime library setup may cause this error. To fix it,
 declare the entry point without `async`:
 
 ```
-fn main() -> Result<i32, ()> { // ok!
-    Ok(1)
+fn main() -> Result<(), ()> { // ok!
+    Ok(())
 }
 ```