fix: read token_account from bumps

[yanCode]

Problem

in this page: https://solana.com/developers/courses/program-security/owner-checks
current rust code in the tutorial, using syntax *ctx.bumps.get("token_account").unwrap() to read token_account from bumps, that now no longer works on anchor-lang = "0.30.1". In fact, this has already been updated in the sample code repository, https://github.com/yanCode/owner-checks/blob/7bda9fb5051976b37ec7cd68aa9cf9595ad087b8/programs/owner-check/src/lib.rs#L49

I personally think while the code repository got updated, the doc has'nt been update accordingly.

Summary of Changes

Update the code as:

 let seeds = &[b"token".as_ref(), &[ctx.bumps.token_account]];