Add explanation for base_url

Co-Authored-By: Adrián Chaves <[email protected]>

Author: AnthonyChouGit

parsel/selector.py

@@ -173,7 +173,7 @@ class Selector(object):
     ``type`` defines the selector type, it can be ``"html"``, ``"xml"`` or ``None`` (default).
     If ``type`` is ``None``, the selector defaults to ``"html"``.
 
-    The ``base_url`` keyword allows setting a URL for the document. This is needed when looking up external entities with relative paths.
+    ``base_url`` allows setting a URL for the document. This is needed when looking up external entities with relative paths.
     See [`lxml` documentation](https://lxml.de/api/index.html) ``lxml.etree.fromstring`` for more information.
     """