Fix undefined variable in APY calculation formula

[ruslan0012]

Description:
Line 111 in PendleMarketAPYCalculation.md references holdYtReturns which isn't defined. Should be ytReturns from line 110.

Current: $\text{ytReturnsAfterFee} = \text{holdYtReturns} \times 95\%$
Fix: $\text{ytReturnsAfterFee} = \text{ytReturns} \times 95\%$

[ruslan0012]

I can submit a fix for this if you'd like, thanks