guide-pointers.md: C sample code should match the Rust version.

Fix rust-lang/rust#17255

src/doc/guide-pointers.md

@@ -505,6 +505,7 @@ As being similar to this C code:
 {
     int *x;
     x = (int *)malloc(sizeof(int));
+    *x = 5;
 
     // stuff happens