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 <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Run-TryBot: hopehook <63023fcaeafd0730eb709923fc3c0482e6977249@golangcn.org>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Reviewed-by: Benny Siegert <05f4aee36fd25d0de2a07789fd6e73398db492cc@gmail.com>
Reviewed-by: hopehook <63023fcaeafd0730eb709923fc3c0482e6977249@golangcn.org>
Reviewed-by: Subham <fe2f57036096eea78c35b2929cb9475f9b23ecaf@gmail.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go