encoding/asn1: fix doc for BitString.At's return value

encoding/asn1: fix doc for BitString.At's return value

Fixes #53287

Change-Id: If983ae34850d9b1b29764156a38628fa53897573
Reviewed-on: https://go-review.googlesource.com/c/go/+/411134
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: hopehook <hopehook@golangcn.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: hopehook <hopehook@golangcn.org>
Reviewed-by: Subham <sarkar.subhams2@gmail.com>