OggOpus draft updates.

Bump version and date for draft-ietf-codec-oggopus-03 submission.

Move more text into figure pre/postamble to fix rendering issues
in the xml2rfc html output. These need to be manually re-indented
in the txt output before submission. :(

Fix resampling frequency choice algorithm, which was missing a word.

Fix some spelling and make some minor enphasis changes.