io: show ErrUnexpectedEOF in ExampleReadAtLeast

Fixes #36245

Change-Id: I10ce50b0cc28b15f4e7be85b8f12cf9d0e4fac96
Reviewed-on: https://go-review.googlesource.com/c/go/+/212404
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/io/example_test.go

@@ -59,7 +59,7 @@ func ExampleCopyN() {
 func ExampleReadAtLeast() {
 	r := strings.NewReader("some io.Reader stream to be read\n")
 
-	buf := make([]byte, 33)
+	buf := make([]byte, 14)
 	if _, err := io.ReadAtLeast(r, buf, 4); err != nil {
 		log.Fatal(err)
 	}
@@ -78,10 +78,9 @@ func ExampleReadAtLeast() {
 	}
 
 	// Output:
-	// some io.Reader stream to be read
-	//
+	// some io.Reader
 	// error: short buffer
-	// error: EOF
+	// error: unexpected EOF
 }
 
 func ExampleReadFull() {