Rollup merge of #23067 - oli-obk:doc_examle_fix, r=alexcrichton

The compiler even tells us this won't work.
```
let mut i = s.len();
while i < 0 { ... }
```

src/libcollections/str.rs

@@ -1086,7 +1086,7 @@ pub trait StrExt: Index<RangeFull, Output = str> {
     ///
     /// let s = "中华Việt Nam";
     /// let mut i = s.len();
-    /// while i < 0 {
+    /// while i > 0 {
     ///     let CharRange {ch, next} = s.char_range_at_reverse(i);
     ///     println!("{}: {}", i, ch);
     ///     i = next;