Editorial: Change the _index_^th element of _captures_

... to `_captures_[_index_ - 1]` in `GetSubstitution`.

Also, in `RegExp.prototype [ @@replace ]`
(the only place that passes a non-empty `_captures_` to `GetSubstitution`),
add a NOTE to underline the fact that
`_captures_[_n_ - 1]` is the `_n_`th capture.

spec.html

@@ -33375,7 +33375,7 @@ <h1>
                     1. If _index_ = 0, then
                       1. Let _refReplacement_ be _ref_.
                     1. Else if _index_ &le; the number of elements in _captures_, then
-                      1. Let _capture_ be the _index_<sup>th</sup> element of _captures_.
+                      1. Let _capture_ be _captures_[_index_ - 1].
                       1. If _capture_ is *undefined*, then
                         1. Let _refReplacement_ be the empty String.
                       1. Else,
@@ -35788,6 +35788,7 @@ <h1>RegExp.prototype [ @@replace ] ( _string_, _replaceValue_ )</h1>
               1. If _capN_ is not *undefined*, then
                 1. Set _capN_ to ? ToString(_capN_).
               1. Append _capN_ as the last element of _captures_.
+              1. NOTE: When _n_ = 1, the preceding step puts the first element into _captures_ (at index 0). More generally, the _n_<sup>th</sup> capture (the characters captured by the _n_<sup>th</sup> set of capturing parentheses) is at _captures_[_n_ - 1].
               1. Set _n_ to _n_ + 1.
             1. Let _namedCaptures_ be ? Get(_result_, *"groups"*).
             1. If _functionalReplace_ is *true*, then