Bump build docs to v15

Author: jeromekelleher

.github/workflows/build-docs.yml

@@ -101,7 +101,7 @@ jobs:
 
       - name: Build docs
         if: steps.docs-cache.outputs.cache-hit != 'true'
-        uses: tskit-dev/.github/.github/actions/build-docs@v14
+        uses: tskit-dev/.github/.github/actions/build-docs@v15
         with:
           pyproject-directory: ${{ matrix.pyproject-directory || '.' }}
           additional-apt-packages: ${{ matrix.additional-apt-packages || '' }}