fix package documentation (#109)

update example code to actual go-git.v4

Author: weirdgiraffe

doc.go

@@ -6,30 +6,31 @@
 // extensions.
 //
 // Small example extracting the commits from a repository:
-//  func ExampleBasic_printCommits() {
-//    r, err := git.NewRepository("https://github.com/src-d/go-git", nil)
-//    if err != nil {
-//    	panic(err)
-//    }
+// func ExampleBasic_printCommits() {
+//   r := git.NewMemoryRepository()
+//   o := &git.CloneOptions{
+//       URL: "https://github.com/src-d/go-git",
+//   }
+//   if err := r.Clone(o); err != nil {
+//       panic(err)
+//   }
 //
-//    if err := r.Pull("origin", "refs/heads/master"); err != nil {
-//    	panic(err)
-//    }
+// 	 iter, err := r.Commits()
+// 	 if err != nil {
+//       panic(err)
+//   }
+//   defer iter.Close()
 //
-//    iter := r.Commits()
-//    defer iter.Close()
+//   for {
+//       commit, err := iter.Next()
+//       if err != nil {
+//           if err == io.EOF {
+//               break
+//           }
+//           panic(err)
+//       }
 //
-//    for {
-//    	commit, err := iter.Next()
-//    	if err != nil {
-//    		if err == io.EOF {
-//    			break
-//    		}
-//
-//    		panic(err)
-//    	}
-//
-//    	fmt.Println(commit)
-//    }
+//   fmt.Println(commit)
 //  }
+// }
 package git // import "gopkg.in/src-d/go-git.v4"