TRPL: minor correction on how chars().nth() work

The previous wording was confusing. While would we need to go through
the whole list just to find the first code point? `chars()` being an
iterator, we only need to walk from the beginning of the list.

src/doc/trpl/strings.md

@@ -115,7 +115,7 @@ You can get something similar to an index like this:
 let dog = hachiko.chars().nth(1); // kinda like hachiko[1]
 ```
 
-This emphasizes that we have to go through the whole list of `chars`.
+This emphasizes that we have to walk from the beginning of the list of `chars`.
 
 ## Slicing