docs: correct USDT calculation and markdown styling

[9oelM]

Problems:

1. If USDT uses 6 decimals, 1 USDT = $1 \times 10^{6}$.
2. Markdown formatting is broken because we're using "*" and "**" without escaping.

[reveloper]

@9oelM thank you for the fix.

Could you please re-address your fix to https://docs.ton.org/v3/guidelines/ton-connect/guidelines/preparing-messages page, because we rebuilt the docs structure.