Merge pull request #2964 from jwood803/patch-1

Update markdown in theme documentation

docs/extensions_dev.md

@@ -162,7 +162,7 @@ A theme is a JupyterLab extension that uses a `ThemeManager` and can be
 loaded and unloaded dynamically.  The package must include all static assets 
 that are referenced by `url()` in its CSS files.  The `url()` paths in a CSS
 file served by the Jupyter server must start with the path 
-`'./lab/api/themes/<foo>/', where `foo` is the normalized name of the 
+`'./lab/api/themes/<foo>/'`, where `foo` is the normalized name of the 
 package.  Scoped packages of the form `@org/name` are normalized to 
 `org-name`.  Other package names are not affected.  Note that `'@import'` paths are still given as relative paths, e.g. (`'@import './foo.css';`).  
 The path to the theme  assets is specified `package.json` under the