Update docs/sandbox.md

Author: amotl

docs/sandbox.md

@@ -13,7 +13,7 @@ git clone https://github.com/panodata/sphinx-design-elements
 cd sphinx-design-elements
 python3 -m venv .venv
 source .venv/bin/activate
-pip install --editable ".[develop,docs,test]"
+pip install --editable '.[develop,docs,test]'
 poe check
 ```