Correct a comment in serializing a CBS_ASN1_BITSTRING

The additional 0 byte represents the fact that it has 0 unused bits. It
is confusing/imprecise to describe it as "padding".

Change-Id: I16aa5397312f614fb510002465c359896a6a6964
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/93028
Commit-Queue: David Benjamin <davidben@google.com>
Auto-Submit: Lily Chen <chlily@google.com>
Commit-Queue: Lily Chen <chlily@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/evp/p_mldsa.cc

diff --git a/crypto/evp/p_mldsa.cc b/crypto/evp/p_mldsa.cc
index 1e83147..3e4c662 100644
--- a/crypto/evp/p_mldsa.cc
+++ b/crypto/evp/p_mldsa.cc
@@ -271,7 +271,7 @@
         !CBB_add_asn1_element(&algorithm, CBS_ASN1_OBJECT, Traits::kOID.data(),
                               Traits::kOID.size()) ||
         !CBB_add_asn1(&spki, &key_bitstring, CBS_ASN1_BITSTRING) ||
-        !CBB_add_u8(&key_bitstring, 0 /* padding */) ||
+        !CBB_add_u8(&key_bitstring, 0 /* no unused bits */) ||
         !Traits::MarshalPublicKey(&key_bitstring, pub) ||
         !CBB_flush(out)) {
       OPENSSL_PUT_ERROR(EVP, EVP_R_ENCODE_ERROR);