Fix mis-spellings of "separating" as "seperating"

Found by Debian's Lintian tool.

code/botlib/aasfile.h

@@ -65,8 +65,8 @@ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 #define FACE_LADDER					2		//ladder
 #define FACE_GROUND					4		//standing on ground when in this face
 #define FACE_GAP					8		//gap in the ground
-#define FACE_LIQUID					16		//face seperating two areas with liquid
-#define FACE_LIQUIDSURFACE			32		//face seperating liquid and air
+#define FACE_LIQUID					16		//face separating two areas with liquid
+#define FACE_LIQUIDSURFACE			32		//face separating liquid and air
 #define FACE_BRIDGE					64		//can walk over this face if bridge is closed
 
 //area contents