fix: mint authority

[yanCode]

Problem

in this page https://solana.com/developers/courses/onchain-development/anchor-cpi
if you run anchor test in the final example, you can see errors:

 "Program log: Instruction: MintTo",
  "Program log: Error: owner does not match",

Summary of Changes

the reason is in this line:

developer-content/content/courses/onchain-development/anchor-cpi.md

Line 555 in 5dc01dd

authority: ctx.accounts.mint.to_account_info(),

it passed an incorrect mint authority, because in InitializeMint, the authority passed with user, which is the signer. but when it call MintTo in CpiContext::new_with_signer, authority is incorrectly passed as mint. which is not the authority of itself, instead the signer, ctx.accounts.initializer.to_account_info(), should be passed in.

Fixes #

Also I see in this PR: https://github.com/solana-foundation/developer-content/pull/571/files. it uses the mint as its own authority. well, i am afriad I can't think this is very good based on two reasons:

1. Security: If the mint were its own authority, anyone who could predict the PDA (which is techincally possible! ) could potentially mint tokens.

2. Security: If the mint were its own authority, anyone who could predict the PDA could potentially mint tokens.

[mikemaccana]

Yep 'initializer` seems right. Re: the other PR, please add your comments to the discussion in that PR - that way we can have it in one place!