Though F is more electronegative than Cl  , Therefore  -I effect of F would be more and CHF3 would be more acidic. But this is not the correct explaination as -M effect also plays role and we know that M effect is considered before I effect.If we compare the conjugate base of both of them , C-^{_{Cl3 and C-F3, Cl has vacant 3d orbital whereas F doesn't. Therefore Cl can exert -M effect due to availability of vacant d orbital but F cannot. -M effect stabilises the negative charge and increases acidic strength. So CHCl3 is more acidic than CHF3.}}