block: Clean up exit_io_context() source code.

This patch fixes a spelling error in a source code comment and removes
superfluous braces in the function exit_io_context().

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Jens Axboe <jaxboe@fusionio.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>