doc/next: improve new(expr) release note

One reader pointed out that the example isn't compelling because
&age would have worked just as well. This CL changes the example
to use a nontrivial expression. Don't nitpick the arithmetic.

For golang#45624

Change-Id: Icc745f5ee7000c1d3559da1388c6a5596c4d1f46
Reviewed-on: https://go-review.googlesource.com/c/go/+/714040
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: adonovan

doc/next/2-language.md

@@ -19,10 +19,14 @@ type Person struct {
 	Age  *int     `json:"age"` // age if known; nil otherwise
 }
 
-func personJSON(name string, age int) ([]byte, error) {
+func personJSON(name string, born time.Time) ([]byte, error) {
 	return json.Marshal(Person{
 		Name: name,
-		Age:  new(age),
+		Age:  new(yearsSince(born)),
 	})
 }
+
+func yearsSince(t time.Time) int {
+	return int(time.Since(t).Hours() / (365.25 * 24)) // approximately
+}
 ```