fixed error string grammar

cmd/playground/wallArea.go

@@ -5,10 +5,10 @@ import "fmt"
 func paintNeeded(width float64, height float64) (paintNeededCalculated float64, anError error) {
 	area := width * height
 	if width < 0.0 {
-		return 0, fmt.Errorf("a width of %0.2f is invalid.", width)
+		return 0, fmt.Errorf("a width of %0.2f is invalid", width)
 	}
 	if height < 0.0 {
-		return 0, fmt.Errorf("a height of %0.2f is invalid.", height)
+		return 0, fmt.Errorf("a height of %0.2f is invalid", height)
 	}
 	return area / 10.0, nil
 }