Fix CVA swap leg-order comment

[cuiyeshuai]

Summary

Correct the leg-order comment in CounterpartyAdjSwapEngine::calculate() to state that leg 0 of a vanilla swap is the fixed leg.

Why

FixedVsFloatingSwap stores the fixed leg at index 0 and the floating leg at index 1. The existing comment stated the opposite, even though the nearby casts correctly treat arguments_.legs[0] as a FixedRateCoupon and arguments_.legs[1] as a FloatingRateCoupon.

Impact

Documentation-only correction; there is no runtime behavior change.

Validation

• Verified the FixedVsFloatingSwap leg accessors and constructor ordering.
• Confirmed the branch is one commit ahead of master and changes only this comment.
• Ran git diff --check locally.

[CLAassistant]

All committers have signed the CLA.

[boring-cyborg]

Thanks for opening this pull request! It might take a while before we look at it, so don't worry if there seems to be no feedback. We'll get to it.