Fix public issue #46: Format description said "3-byte offset"
instead of "4-byte offset" for the longest copies.

Also fix an inconsistency in the heading for section 2.2.3.
Both patches by Patrick Pelletier.

R=csilvers


git-svn-id: https://snappy.googlecode.com/svn/trunk@45 03e5f5b5-db94-4691-08a0-1a8bf15f6143