Consistent sample usage command in README.md

[gagangupt16]

Environment details

• OS: All
• Node.js version: All
• npm version: All
• @google-cloud/spanner version: All

Steps to reproduce

1. Read README.md of samples folder.
2. Some examples start with node file_name.js ... and some with node samples/file_name.js...
3. Follow steps inBefore you begin section,
4. For samples, where it samples/ is not prefixed, it will give MODULE_NOT_FOUND error.

Making sure to follow these steps will guarantee the quickest resolution possible.

Thanks!