readme mint_token name, clearer rate limit start, typo on limit error

[mooncitydev]

what changed

• readme: the on-chain instruction is mint_token (singular); the docs said mint_tokens.
• add_minter: set remaining_amount to capacity and last_refill_time to the current unix timestamp so new minters start with a full bucket instead of depending on last_refill_time == 0 and a huge synthetic refill.
• mint_token: fix the user-facing error string typo (exceeeded to exceeded).
• tests: rename a duplicate it description so the suite reads clearer in logs.

testing

Could not run anchor test here (no anchor/rust toolchain on this machine). Worth a quick anchor test on your side before merge.