404 caused if branch name contains upper case chars

[KengoTODA]

https://github.com/spotbugs/spotbugs/runs/3530663018#step:2:6

UnhandledPromiseRejectionWarning: Error: Unexpected status code 404 with body: {"detail":"Not found."}
at /home/runner/work/_actions/KengoTODA/readthedocs-action/main/webpack:/rtd-bot/src/rtd.ts:141:1
at processTicksAndRejections (internal/process/task_queues.js:93:5)

it seems that RTD server requires lower case, because the URL of version on the site is like https://readthedocs.org/dashboard/spotbugs/version/kengotoda-patch-1/edit/
even when the branch is named as KengoTODA-patch-1.