fix(spanner): fix layout of godoc for Encoder and Decoder

[yfuruyama]

Fix: #4127

I added an additional indent to code sample to fix the layout issue for Encoder/Decoder.

This is a screenshot for the godoc for the fixed code (I used the godoc command to check the fix, so the style of godoc is an old one):

[hengfengli]

LGTM. Thanks for the fix!

[codyoss]

nit: It would be nice to turn these into a first class godoc example. Here is an example from the storage library:

• Godoc: https://pkg.go.dev/cloud.google.com/go/storage#example-ACLHandle.Delete
• Code:

  google-cloud-go/storage/example_test.go

  Lines 527 to 537 in a5bbdb5

  	func ExampleACLHandle_Delete() {
  	ctx := context.Background()
  	client, err := storage.NewClient(ctx)
  	if err != nil {
  	// TODO: handle error.
  	}
  	// No longer grant access to the bucket to everyone on the Internet.
  	if err := client.Bucket("my-bucket").ACL().Delete(ctx, storage.AllUsers); err != nil {
  	// TODO: handle error.
  	}
  	}

[hengfengli]

I am not sure if this is a good case for that. There are two reasons to put in this way:

• The first class godoc example seems to be a main function to guide users how to use the api. Here we just want to show how to implement the interfaces.
• The example can be close to the code. Probably better in the comment instead of jumping to another file.

[yfuruyama]

Thanks for suggestion! This example contains a type definition and method definition, so if I understand correctly, we need a separate file to turn this into first class godoc example.

For example, sort package has a similar example for Example (SortWrapper) by this code. I'm wondering if I should do similar thing for this example.

[hengfengli]

I'll merge this in first. We can improve the doc later if necessary.