Change start to #[start] in some diagnosis

They refer to a function with the `start` attribute, but not necessarily named `start`.

compiler/rustc_error_codes/src/error_codes/E0647.md

@@ -7,7 +7,7 @@ Erroneous code example:
 
 #[start]
 fn start(_: isize, _: *const *const u8) -> isize where (): Copy {
-    //^ error: start function is not allowed to have a where clause
+    //^ error: `#[start]` function is not allowed to have a where clause
     0
 }
 ```