Fix typo labeling attrs type bool instead of list

Author: whodatninja

docs/topics/link-extractors.rst

@@ -88,7 +88,7 @@ SgmlLinkExtractor
     :param attrs: list of attributes which should be considered when looking
         for links to extract (only for those tags specified in the ``tags``
         parameter). Defaults to ``('href',)``
-    :type attrs: boolean
+    :type attrs: list
 
     :param canonicalize: canonicalize each extracted url (using
         scrapy.utils.url.canonicalize_url). Defaults to ``True``.