README.md: Fix .bazelversion location

[claymation]

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

[google-cla[bot]]

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[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.