Address PR comments on AudioDecoderConfig description

Fixes the grammar ("assumed to a" -> "assumed to be an"), updates
the AudioSpecificConfig table reference from 1.15 to 1.19, and
adds "format" after {{AacBitstreamFormat/aac}} as requested by the
reviewer.

Co-authored-by: Djuffin <132926+Djuffin@users.noreply.github.com>

Author: google-labs-jules[bot]

aac_codec_registration.src.html

@@ -84,9 +84,9 @@
 AudioDecoderConfig description {#audiodecoderconfig-description}
 ================================================================
 
-If {{AudioDecoderConfig/description}} is present, it is assumed to a
-`AudioSpecificConfig` as defined in [[iso14496-3]] section 1.6.2.1, Table 1.15,
-and the bitstream is assumed to be in {{AacBitstreamFormat/aac}}.
+If {{AudioDecoderConfig/description}} is present, it is assumed to be an
+`AudioSpecificConfig` as defined in [[iso14496-3]] section 1.6.2.1, Table 1.19,
+and the bitstream is assumed to be in {{AacBitstreamFormat/aac}} format.
 
 If the {{AudioDecoderConfig/description}} is not present, the bitstream is
 assumed to be in {{AacBitstreamFormat/adts}} format.