auto merge of #17512 : nodakai/rust/fix-ptr-guide, r=alexcrichton

Fix #17255

This is such a trivial change.  Devs: perhaps you might want to omit to run `make check` on Travis.

Author: bors

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