fix example code for E0617

Author: ibraheemdev

compiler/rustc_error_codes/src/error_codes/E0617.md

@@ -3,12 +3,13 @@ Attempted to pass an invalid type of variable into a variadic function.
 Erroneous code example:
 
 ```compile_fail,E0617
+# use std::os::raw::{c_char, c_int};
 extern "C" {
-    fn printf(c: *const i8, ...);
+    fn printf(format: *const c_char, ...) -> c_int;
 }
 
 unsafe {
-    printf(::std::ptr::null(), 0f32);
+    printf("%f\n\0".as_ptr() as _, 0f32);
     // error: cannot pass an `f32` to variadic function, cast to `c_double`
 }
 ```
@@ -21,10 +22,12 @@ to import from `std::os::raw`).
 In this case, `c_double` has the same size as `f64` so we can use it directly:
 
 ```no_run
+# use std::os::raw::{c_char, c_int};
 # extern "C" {
-#     fn printf(c: *const i8, ...);
+#     fn printf(format: *const c_char, ...) -> c_int;
 # }
+
 unsafe {
-    printf(::std::ptr::null(), 0f64); // ok!
+    printf("%f\n\0".as_ptr() as _, 0f64); // ok!
 }
 ```