bytes: make ExampleTrimLeft and ExampleTrimRight match

ExampleTrimLeft was inexplicably complex.

Change-Id: I13ca81bdeba728bdd632acf82e3a1101d29b9f39
Reviewed-on: https://go-review.googlesource.com/78111
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/bytes/example_test.go

@@ -383,9 +383,9 @@ func ExampleMap() {
 }
 
 func ExampleTrimLeft() {
-	fmt.Print(string(bytes.TrimLeft([]byte("+ 005400"), "+0 ")))
+	fmt.Print(string(bytes.TrimLeft([]byte("453gopher8257"), "0123456789")))
 	// Output:
-	// 5400
+	// gopher8257
 }
 
 func ExampleTrimLeftFunc() {