Update README.rst - Fix typo in spanner lib

[ghadlich]

<Fixes Typo - no issue created as trivial change>

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• [N/A] Ensure the tests and linter pass
• [N/A] Code coverage does not decrease (if any source code was changed)
• [N/A] Appropriate docs were updated (if necessary)

Fixes #176 🦕

[ghadlich]

Will update commit message