fix formula in comment (#4006)

Author: joepetrowski

node/runtime/src/impls.rs

@@ -61,7 +61,7 @@ impl<C: Get<Balance>> Convert<Weight, Balance> for LinearWeightToFee<C> {
 
 /// Update the given multiplier based on the following formula
 ///
-///   diff = (target_weight - previous_block_weight)
+///   diff = (previous_block_weight - target_weight)
 ///   v = 0.00004
 ///   next_weight = weight * (1 + (v . diff) + (v . diff)^2 / 2)
 ///