wire: fix example in the ProvideBaz section (google/go-cloud#229)

README.md

@@ -77,8 +77,8 @@ type Baz struct {
 
 // ProvideBaz returns a value if Bar is not zero.
 func ProvideBaz(ctx context.Context, bar Bar) (Baz, error) {
-	if bar == 0 {
-		return 0, errors.New("cannot provide baz when bar is zero")
+	if bar.X == 0 {
+		return Baz{}, errors.New("cannot provide baz when bar is zero")
 	}
 	return Baz{X: bar.X}, nil
 }