ICS20: Correction to the denom in the sequence diagram

This PR addresses one imprecision in ICS20 README that may cause confusion in otherwise very helpful sequence diagram example. In the diagram illustrating multi-chain token transfer, there is a mistake in the denoms (in the last transaction): - `B` should be burning `transfer/ChannelToA/denom` (instead of `transfer/ChannelToB/transfer/ChannelToA/denom`) - `B` should be sending to `A` `transfer/ChannelToA/denom` (instead of `transfer/ChannelToB/transfer/ChannelToA/denom`) - `A` should be unlocking `denom` (instead of `transfer/channelToA/denom`)