Fix off by one error

https://news.ycombinator.com/item?id=23859421

Author: steveklabnik

posts/2020-07-16-Rust-1.45.0.md

@@ -103,7 +103,7 @@ In the end, the decision was made to do this:
 
 This is very similar to array access, for example:
 
-* `array[i]` will check to make sure that `array` has at least `i` elements.
+* `array[i]` will check to make sure that `array` has at least `i + 1` elements.
 * You can use `unsafe { array.get_unchecked(i) }` to skip the check.
 
 So, what's a saturating cast? Let's look at a slightly modified example: