strings: Update Trim example.

Use an input which better shows that behaviour of the function. Only leading
and trailing runes are trimed, not intermediate ones.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/42390043
diff --git a/src/pkg/strings/example_test.go b/src/pkg/strings/example_test.go
index ccfc417..7350131 100644
--- a/src/pkg/strings/example_test.go
+++ b/src/pkg/strings/example_test.go
@@ -150,8 +150,8 @@
 }
 
 func ExampleTrim() {
-	fmt.Printf("[%q]", strings.Trim(" !!! Achtung !!! ", "! "))
-	// Output: ["Achtung"]
+	fmt.Printf("[%q]", strings.Trim(" !!! Achtung! Achtung! !!! ", "! "))
+	// Output: ["Achtung! Achtung"]
 }
 
 func ExampleMap() {