os: Add example for Expand function.

Change-Id: I581492c29158e57ca2f98b75f47870791965a7ff
Reviewed-on: https://go-review.googlesource.com/81155
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/example_test.go

@@ -82,6 +82,24 @@ func init() {
 	os.Unsetenv("GOPATH")
 }
 
+func ExampleExpand() {
+	mapper := func(placeholderName string) string {
+		switch placeholderName {
+		case "DAY_PART":
+			return "morning"
+		case "USER":
+			return "Gopher"
+		}
+
+		return ""
+	}
+
+	fmt.Println(os.Expand("Good ${DAY_PART}, $USER!", mapper))
+
+	// Output:
+	// Good morning, Gopher!
+}
+
 func ExampleExpandEnv() {
 	fmt.Println(os.ExpandEnv("$USER lives in ${HOME}."))