Don't have UB in E0617 example

This uses an actual format string instead of passing null pointer.

Closes rust-lang#86908.

Author: syvb

compiler/rustc_error_codes/src/error_codes/E0617.md

@@ -4,11 +4,11 @@ Erroneous code example:
 
 ```compile_fail,E0617
 extern "C" {
-    fn printf(c: *const i8, ...);
+    fn printf(c: *const u8, ...) -> std::os::raw::c_int;
 }
 
 unsafe {
-    printf(::std::ptr::null(), 0f32);
+    printf("my favourite number is %f".as_ptr(), 4_f32);
     // error: cannot pass an `f32` to variadic function, cast to `c_double`
 }
 ```
@@ -22,9 +22,9 @@ In this case, `c_double` has the same size as `f64` so we can use it directly:
 
 ```no_run
 # extern "C" {
-#     fn printf(c: *const i8, ...);
+#     fn printf(c: *const u8, ...) -> std::os::raw::c_int;
 # }
 unsafe {
-    printf(::std::ptr::null(), 0f64); // ok!
+    printf("my favourite number is %f".as_ptr(), 4_f64); // ok!
 }
 ```