Update index.bs

Co-authored-by: Felicia Lim <flim@google.com>

index.bs

@@ -1896,7 +1896,7 @@ where <code>xxx</code> is three digits to indicate the value of the [=primary_pr
 
 ### Decapsulating an ISOBMFF IAMF File with a Single Track ### {#isobmff-decapsulation-singletrack}
 
-This section provides a guideline for IAMF parsers reconstructing [=IA Sequence=] from an IAMF file with a single track, as follows.
+This section provides a guideline for IAMF parsers reconstructing an [=IA Sequence=] from an IAMF file with a single track, as follows.
 
 1. The [=configOBUs=] from the [=IASampleEntry=] are placed at the beginning of the [=IA Sequence=]. These are the [=Descriptors=].