PDA Constraints

#[derive(Accounts)]
#[instruction(course_id: String)]
pub struct CreateCourse<'info> {
    #[account(
        init,
        payer = authority,
        space = 8 + Course::INIT_SPACE,
        seeds = [b"course", course_id.as_bytes()],
        bump
    )]
    pub course: Account<'info, Course>,
    #[account(mut)]
    pub authority: Signer<'info>,
}