rust: bump doesn't update library version in README.md

[JoeWang1127]

After running librarian bump -all, the version in README.md is not updated and the presubmit failed.

See googleapis/google-cloud-rust#5142, commit 27cd1a3 and 074d7ae for more details.

[dbolduc]

Hm, I opened this before #3224

I guess it was not fixed.