path: add path.Dir example with trailing slash

Change-Id: I143203a9dcf9a4da0e53a3aab6e370244b849296
Reviewed-on: https://go-review.googlesource.com/62270
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/path/example_test.go

@@ -47,11 +47,15 @@ func ExampleClean() {
 func ExampleDir() {
 	fmt.Println(path.Dir("/a/b/c"))
 	fmt.Println(path.Dir("a/b/c"))
+	fmt.Println(path.Dir("/a/"))
+	fmt.Println(path.Dir("a/"))
 	fmt.Println(path.Dir("/"))
 	fmt.Println(path.Dir(""))
 	// Output:
 	// /a/b
 	// a/b
+	// /a
+	// a
 	// /
 	// .
 }