fix: use buyer_ata instead of buyer_token_account in mint instruction

[0xRektified]

Issue: Incorrect account name in MintTo instruction

Description

In the mint() function code example, the MintTo instruction references buyer_token_account, but this account name doesn't match the actual account defined in the MintTokens struct.

Location

File: content/spl-token-sale-tutorial/spl-token-sale-tutorial.md

Current Code

let mint_to_instruction = MintTo {
    mint: ctx.accounts.mint.to_account_info(),
    to: ctx.accounts.buyer_token_account.to_account_info(),  // ❌ Incorrect
    authority: ctx.accounts.mint.to_account_info(),
};
Expected Code
let mint_to_instruction = MintTo {
    mint: ctx.accounts.mint.to_account_info(),
    to: ctx.accounts.buyer_ata.to_account_info(),  // ✅ Correct
    authority: ctx.accounts.mint.to_account_info(),
};

Reason

The MintTokens struct (defined later in the tutorial) uses buyer_ata as the account name:

#[derive(Accounts)]
pub struct MintTokens<'info> {
    // ...
    pub buyer_ata: Account<'info, TokenAccount>,
    // ...
}

[0xRektified]

Hi @RareSkills @jeffreyscholz , gentle follow-up in case this PR slipped through. Thank you!