regexp: fix typo in the overview
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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Trust: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>

CAFxX/go,CAFxX/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,golang/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,golang/go