Improve substring description (mdn#22559)

The previous version of the text made it reasonable to assume that if the text would be "Mozilla" and the indexes `(1, 3)`, the end result will be the character `z`,
i.e. the string "between the start and end indexes".

Since that would be very unusual, many users might instead assume that it would result in `ozi`. But that would also be incorrect.

This change makes it clear that the function includes the first index and excludes the second.

files/en-us/web/javascript/reference/global_objects/string/substring/index.md

@@ -12,7 +12,7 @@ browser-compat: javascript.builtins.String.substring
 
 {{JSRef}}
 
-The **`substring()`** method returns the part of the `string` between the start and end indexes, or to the end of the string.
+The **`substring()`** method returns the part of the `string` from the start index up to and excluding the end index, or to the end of the string if no end index is supplied.
 
 {{EmbedInteractiveExample("pages/js/string-substring.html")}}