io/ioutil: fix example test for WriteFile to allow it to run in the playground

The example for WriteFile assumed the existence of a testdata/ directory, which is not present on the playground. The example now writes the file to the current working directory, rather than to testdata/.

Fixes #32916

Change-Id: I577caac7e67ba9d9941b2dd19346ad5ff61e78d9
GitHub-Last-Rev: 40f14e0
GitHub-Pull-Request: #43757
Reviewed-on: https://go-review.googlesource.com/c/go/+/284452
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Trust: Bryan C. Mills <[email protected]>

Author: pantonshire

src/io/ioutil/example_test.go

@@ -125,7 +125,7 @@ func ExampleReadFile() {
 
 func ExampleWriteFile() {
 	message := []byte("Hello, Gophers!")
-	err := ioutil.WriteFile("testdata/hello", message, 0644)
+	err := ioutil.WriteFile("hello", message, 0644)
 	if err != nil {
 		log.Fatal(err)
 	}