Wrap RTD git update-index line

.readthedocs.yml

@@ -8,7 +8,9 @@ build:
     pre_install:
       # Avoid `git` treating the directory is dirty due to RTD changes.
       # ref: https://docs.readthedocs.io/en/stable/build-customization.html#avoid-having-a-dirty-git-index
-      - git update-index --assume-unchanged continuous_integration/environment-doc.yml docs/conf.py
+      - > git update-index --assume-unchanged
+          continuous_integration/environment-doc.yml
+          docs/conf.py
       # If we missed any, error and list the changed files.
       - git diff --stat --exit-code