broken links fixed

20: Sell Coins for Bitcoins (currency trade offer)
https://github.com/mastercoin-MSC/spec#sell-coins-for-bitcoins

should be linked to 
https://github.com/mastercoin-MSC/spec#sell-mastercoins-for-bitcoins

likewise

22: Purchase Coins with Bitcoins (accept currency trade offer)
https://github.com/mastercoin-MSC/spec#purchase-coins-with-bitcoins

should be linked to

https://github.com/mastercoin-MSC/spec#purchase-mastercoins-with-bitcoins