Updating the README in Python Spanner to include instructions for running Python samples

[aakashanandg]

Update the README in the Python Spanner repository to reflect the correct steps for running Python samples. Previously, the README instructed users to clone the python-docs-samples.git repository, but the content has since been relocated, as outlined here: https://github.com/GoogleCloudPlatform/python-docs-samples/tree/main/spanner.

[aakashanandg]

Pull request: googleapis/python-spanner#1266

[chalmerlowe]

This issue was moved from the following repo as part of the monorepo migration: googleapis/python-spanner