Fix `setup-gradle` step in `build-docs` action

[austin-denoble]

Problem

I added a new action and workflow for building and generating client reference docs. Upon running the workflow, I realized I forgot the /actions/ part of the setup-gradle action. 🤦‍♂️

https://github.com/pinecone-io/pinecone-java-client/actions/runs/9020304652/job/24785162144

Solution

Update the uses field for the Setup Gradle step to point at the proper external action.

Type of Change

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [X] Infrastructure change (CI configs, etc)
• [ ] Non-code change (docs, etc)
• [ ] None of the above: (explain here)

Test Plan

Try re-running the action once it's pushed to main