fix: usr: DSYNC record incorrectly used two octets for the Scheme Field

When creating the `DSYNC` record from a structure, `uint16_tobuffer` was used instead of `uint8_tobuffer` when adding the scheme, causing a `DSYNC` record that was one octet too long. This has been fixed.

Closes #5711

Merge branch '5711-dsync_fromstruct-produces-an-invalid-record' into 'main'

See merge request isc-projects/bind9!11477