docs: update the README instructions for running Spanner samples

[aakashanandg]

• Issue: Updating the README in Python Spanner to include instructions for running Python samples #1265
• Ensure the tests and linter pass : Yes
• Code coverage does not decrease : This change will not impact code coverage
• Appropriate docs were updated: Yes

Fixes #<1265> 🦕

[olavloite]

The change itself LGTM, but the pull request title needs to be changed. We use conventional commits to generate the release notes. This means that:

1. All commits and pull request titles should start with a commit type (feat:, docs:, fix:, etc.) In this case, the type should be docs:
2. The rest of the header automatically goes into the release notes, so it should preferably be worded in a way that it reads naturally for someone reading it after it has been released. So in this case I would suggest docs: update the README instructions for running Spanner samples

[aakashanandg]

The change itself LGTM, but the pull request title needs to be changed. We use conventional commits to generate the release notes. This means that:

1. All commits and pull request titles should start with a commit type (feat:, docs:, fix:, etc.) In this case, the type should be docs:
2. The rest of the header automatically goes into the release notes, so it should preferably be worded in a way that it reads naturally for someone reading it after it has been released. So in this case I would suggest docs: update the README instructions for running Spanner samples

Got it! I've updated the PR description to follow the conventional commits guideline.