Fix invariant check in token-swap swap function

[Zentex1337]

Summary

• Fixed the invariant check in swap_exact_tokens_for_tokens.rs at line 137
• Changed pool_account_a.amount * pool_account_a.amount to pool_account_a.amount * pool_account_b.amount
• This correctly verifies the constant product invariant (x * y = k) used in the CPAMM

Test plan

• The change is straightforward and corrects an obvious typo
• The invariant check should now properly compare against the product of both token amounts

Fixes #477