doc/encoder/aac: Remove -cutoff trick

Fix Ticket #2686 comment 155.

Trick obsoleted since 72dabdfc58064fcb249f26fc7afca2221f18a832.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>