Fix lint error formatting

OpenRA.Mods.Common/Lint/CheckMapCordon.cs

@@ -21,7 +21,7 @@ namespace OpenRA.Mods.Common.Lint
 				|| map.Bounds.Right == map.MapSize.X || map.Bounds.Bottom == map.MapSize.Y)
 				emitError("This map does not define a valid cordon.\n"
 					+ "A one cell (or greater) border is required on all four sides "
-					+ "between the playable bounds and the map edges");
+					+ "between the playable bounds and the map edges.");
 		}
 	}
 }