Replace with saturating sub (#930)

## Describe your changes

1. The auction logic is now using saturating subtraction.
2. Fixed a typo

pallets/ocex/src/lib.rs

@@ -1530,7 +1530,8 @@ pub mod pallet {
 										);
 										let fee_to_be_burnt =
 											Percent::from_percent(distribution.burn_ration) * fees;
-										let fee_to_be_distributed = fees - fee_to_be_burnt;
+										let fee_to_be_distributed =
+											fees.saturating_sub(fee_to_be_burnt);
 										// Burn the fee
 										let imbalance = T::NativeCurrency::burn(
 											fee_to_be_burnt.saturated_into(),
@@ -2118,11 +2119,12 @@ pub mod pallet {
 				let _ = T::NativeCurrency::unreserve(&bidder, total_bidder_reserve_balance);
 				let amount_to_be_burnt =
 					Percent::from_percent(fee_config.burn_ration) * total_bidder_reserve_balance;
-				let trasnferable_amount = total_bidder_reserve_balance - amount_to_be_burnt;
+				let transferable_amount =
+					total_bidder_reserve_balance.saturating_sub(amount_to_be_burnt);
 				T::NativeCurrency::transfer(
 					&bidder,
 					&fee_config.recipient_address,
-					trasnferable_amount,
+					transferable_amount,
 					ExistenceRequirement::KeepAlive,
 				)?;
 
@@ -2139,7 +2141,7 @@ pub mod pallet {
 				Self::deposit_event(Event::<T>::AuctionClosed {
 					bidder,
 					burned: Compact::from(amount_to_be_burnt),
-					paid_to_operator: Compact::from(trasnferable_amount),
+					paid_to_operator: Compact::from(transferable_amount),
 				})
 			}
 			Ok(())