doc: Fix build_doc action

Signed-off-by: Jonas K. <[email protected]>

Author: EverythingElseWasAlreadyTaken

.github/workflows/build_docs.yml

@@ -17,5 +17,5 @@ jobs:
       - uses: ammaraskar/[email protected]
         with:
           pre-build-command: "pip install -r requirements.txt"
-          build-command: 'make html SPHINXOPTS="-W --keep-going -n"'
+          build-command: 'make html SPHINXOPTS+="-W --keep-going -n"'
           docs-folder: "docs/"