Typo correction and fix for possible detachment lockup

This patch fixes a typo in the scenario instructions, and fixes an issue where a detachment could be legally placed next to an empty fort, but if your opponent moved a unit into the fort, the detachment would be required to conduct a mandatory attack. Detachments cannot attack, however, and the game would be stuck at that point. The fix checks at the start of each attack phase to see if there are any detachments with mandatory attacks, and if there are, they are recalled and a message explaining why is displayed to the player(s).