Fix samples readme reference

[jsoref]

Thanks for stopping by to let us know something could be better!

Environment details

• synthtool version: master

Steps to reproduce

1. Read the sentence in https://github.com/googleapis/nodejs-dlp/blob/master/README.md#samples
2. Determine that the grammar is confusing and that the message is wrong.
3. File Fix samples readme reference nodejs-dlp#423
4. Told to file an issue

[jsoref]

@bcoe:
Afaiu, you're thinking about https://github.com/googleapis/synthtool/blob/master/synthtool/gcp/templates/node_library/samples/README.md which is a separate file.

If you mean that the message itself is talking about those files, then the change should be of the form:

## Samples

-Samples are in the [`samples/`](https://github.com/{{  metadata['repo']['repo'] }}/tree/master/samples) directory. The samples' `README.md`
-has instructions for running the samples.
+Samples are in the [`samples/`](https://github.com/{{  metadata['repo']['repo'] }}/tree/master/samples) directory. Each sample's `README.md`
+has instructions for running its sample.

I'm happy to make a PR for that if that's the intention. Or if the intention is closer to my initial interpretation, I'm happy to make a PR for that.

[sofisl]

Addressed in #872, will hold off on merging until after fix-it week