move code examples

https://pkg.go.dev/github.com/go-logr/[email protected] did not pick them up when
placed in the "example" directory.

Author: pohly

example/example_test.go example_test.go

@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-package main
+package stdr_test
 
 import (
 	"errors"