auto merge of #16614 : cdwort/rust/pointer-guide-typo, r=steveklabnik

This appears to be a minor typo. This example implies that x is mutable otherwise the compiler would error on the line before the comment implies.

Please let me know if I'm missing something - I'd love to learn what I got wrong!

src/doc/guide-pointers.md

@@ -332,7 +332,7 @@ let z = &x;
 Mutable ones, however, are not:
 
 ```{rust,ignore}
-let x = 5i;
+let mut x = 5i;
 let y = &mut x;
 let z = &mut x; // error: cannot borrow `x` as mutable more than once at a time
 ```