Fix function name

Author: kazukousen

pperr.go

@@ -34,16 +34,16 @@ func Fprint(w io.Writer, err error) {
 }
 
 func FprintFunc(w io.Writer, err error, puts Printer) {
-	for _, e := range extractErrorSet(err, nil) {
+	for _, e := range extractErrorSets(err, nil) {
 		puts(w, e.Error, e.Frames, e.Parent)
 	}
 }
 
-func ExtractErrorSet(err error) []ErrorSet {
-	return extractErrorSet(err, nil)
+func ExtractErrorSets(err error) ErrorSets {
+	return extractErrorSets(err, nil)
 }
 
-func extractErrorSet(err error, parent Frames) []ErrorSet {
+func extractErrorSets(err error, parent Frames) []ErrorSet {
 	if err == nil {
 		return nil
 	}

pperr_test.go

@@ -48,7 +48,7 @@ func TestExtractErrorSet(t *testing.T) {
 	err := f1(true)
 
 	var buf strings.Builder
-	for _, e := range pperr.ExtractErrorSet(err) {
+	for _, e := range pperr.ExtractErrorSets(err) {
 		pperr.DefaultPrinter(&buf, e.Error, e.Frames, e.Parent)
 	}
 	actual := buf.String()