The documentation states that here 0 should be used for read-only and
1 for a writable buffer. AVIO_FLAG_WRITE however is 2, while it works
due to the way the flag is handled internally, it is still wrong
according to the documentation.

Additionally it makes it seem as if the AVIO_FLAG_* values could be used
here, which is actually not true, as when AVIO_FLAG_READ would be used
here it would create a writable buffer as AVIO_FLAG_READ is defined as 1.
Signed-off-by: Anton Khirnov <anton@khirnov.net>