remove unwanted quotes

Author: christophdb

docs/customization/custom-css.md

@@ -9,7 +9,7 @@ services:
     environment:
       # ...
       - SEAHUB__ENABLE_BRANDING_CSS=True
-      - SEAHUB__BRANDING_CSS="/custom/custom.css"
+      - SEAHUB__BRANDING_CSS=/custom/custom.css
     volumes:
       - ./your-custom-styles.css:/shared/seafile/seahub-data/custom/custom.css:ro
 ```