oggopus: use 'invalid' to match other occurances and RFC 6716.

Suggestion from mark4o.

doc/draft-ietf-codec-oggopus.xml

@@ -182,7 +182,7 @@ All of the Opus packets in a single Ogg packet MUST be constrained to have the
  same duration.
 A decoder SHOULD treat any Opus packet whose duration is different from that of
  the first Opus packet in an Ogg packet as if it were an Opus packet with an
- illegal TOC sequence.
+ invalid TOC sequence.
 </t>
 <t>
 The coding mode (SILK, Hybrid, or CELT), audio bandwidth, channel count,
@@ -199,7 +199,7 @@ The first audio data page SHOULD NOT have the 'continued packet' flag set
 Packets MUST be placed into Ogg pages in order until the end of stream.
 Audio packets MAY span page boundaries.
 A decoder MUST treat a zero-octet audio data packet as if it were an Opus
- packet with an illegal TOC sequence.
+ packet with an invalid TOC sequence.
 The last page SHOULD have the 'end of stream' flag set, but implementations
  need to be prepared to deal with truncated streams that do not have a page
  marked 'end of stream'.