regexp: fix typo in the overview

Correct the slice expression in the description of Index functions.

Change-Id: I97a1b670c4c7e600d858f6550b647f677ef90b41
Reviewed-on: https://go-review.googlesource.com/c/go/+/360058


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Trust: Daniel Martí <mvdan@mvdan.cc>