README.md: Fix .bazelversion location

[claymation]

The Python and Go implementations both look for .bazelversion only in the workspace root.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[claymation]

Signed CLA after the fact, how can I re-run the check?

[albertocavalcante]

@claymation You should be able to click on the "details" for the cla/google action and then there will be a rescan option as below:

🔁 New Contributors: Update this check after signing the CLA by clicking here.

[albertocavalcante]

@claymation do you still intend to proceed with this PR?

[claymation]

@albertocavalcante yes, rebased.