Update index.bs

index.bs

@@ -481,7 +481,7 @@ NOTE: Because of coding dependency, discarding a sample can sometimes mean decod
 
 NOTE: This means that if one of the values is set to the number of samples in the [=Audio Frame OBU=] (i.e., [=num_samples_per_frame=]), the other value is set to 0.
 
-- When [=num_samples_to_trim_at_start=] is non-zero, all [=Audio Frame OBU=]s with the same [=audio_substream_id=], and preceding this OBU back until the [=Codec Config OBU=] defining this [=Audio Substream=], SHALL have their [=num_samples_to_trim_at_start=] field equal to the number of samples in the corresponding Audio Frame OBU (i.e., [=num_samples_per_frame=]).
+- When [=num_samples_to_trim_at_start=] is non-zero, all [=Audio Frame OBU=]s with the same [=audio_substream_id=], and preceding this OBU back until the [=Codec Config OBU=] defining this [=Audio Substream=], SHALL have their [=num_samples_to_trim_at_start=] field equal to the number of samples in the corresponding [=Audio Frame OBU=] (i.e., [=num_samples_per_frame=]).
 - When [=num_samples_to_trim_at_end=] is non-zero in an [=Audio Frame OBU=], there SHALL be no subsequent [=Audio Frame OBU=] with the same [=audio_substream_id=] until a non-redundant [=Codec Config OBU=] defining an [=Audio Substream=] with the same [=audio_substream_id=].
 
 <dfn noexport>obu_extension_flag</dfn> indicates whether the [=extension_header_size=] field is present. If it is set to 0, the [=extension_header_size=] field SHALL NOT be present. Otherwise, the [=extension_header_size=] field SHALL be present.