update book code

eBook/exercises/chapter_10/celsius.go

@@ -0,0 +1,19 @@
+// celsius.go
+package main
+
+import (
+	"fmt"
+	"strconv"
+)
+
+type Celsius float64
+
+func (c Celsius) String() string {
+	return "The temperature is: " +  strconv.FormatFloat(float64(c),'f', 1, 32) + " °C"
+}
+
+func main() {
+	var c Celsius = 18.36
+	fmt.Println(c)
+}
+// The temperature is: 18.4 °C