Update build-docs.yaml (#775)

* Update build-docs.yaml

* Update build-docs.yaml

* fix repo branch to clone

.github/workflows/build-docs.yaml

@@ -34,7 +34,7 @@ on:
       version:
         description: 'The release tag or branch to build and deploy 
         (e.g., "2.8.0", "2.8.0-docs-hotfix"). 
-        Use `unreleased` to build directly from the main branch of `PrefectHQ/prefect`.'
+        Use `unreleased` to build directly from the 2.x branch of `PrefectHQ/prefect`.'
         type: string
         required: true
         default: 'unreleased'
@@ -96,7 +96,7 @@ jobs:
       run: |
         version="${{ env.version }}"
         if [[ $version == "unreleased" ]]; then
-          git clone --depth 1 https://github.com/PrefectHQ/prefect.git prefect_source
+          git clone --branch 2.x --depth 1 https://github.com/PrefectHQ/prefect.git prefect_source
         else
           git clone --branch "$version" --depth 1 https://github.com/PrefectHQ/prefect.git prefect_source
         fi