Documentation: seccomp: Fix typo in user notification

The close on exec flag is O_CLOEXEC, not O_EXEC. This patch just fixes
the typo.

Suggested-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Rodrigo Campos <rodrigo@kinvolk.io>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Fixes: 0ae71c77 ("seccomp: Support atomic "addfd + send reply"")
Link: https://lore.kernel.org/r/20210702151927.263402-1-rodrigo@kinvolk.io