Fix heading indentation on righthand nav in c.g.c. reference docs

[alicejli]

parent bug: b/278903399
Corresponds to [R9] Confusing ‘On This Page’ Indentation on go/cloud-java-rad-refactor and slide 20 on go/cloud-java-rad-visual-review.

For example on: https://cloud.google.com/java/docs/reference/google-cloud-apikeys/latest/overview

APIKeysClient is outdented even though it is a child of com.google.api.apikeys.v2.

This will be a two-part fix, with this being the doclet change and another template change needed in the https://github.com/googleapis/doc-pipeline repo.