Merge pull request matplotlib#23258 from rcomer/purge-blue-book

DOC: remove Blue Book url

Author: story645

lib/matplotlib/backend_bases.py

@@ -812,12 +812,9 @@ def get_dashes(self):
         """
         Return the dash style as an (offset, dash-list) pair.
 
-        The dash list is a even-length list that gives the ink on, ink off in
-        points.  See p. 107 of to PostScript `blue book`_ for more info.
+        See `.set_dashes` for details.
 
         Default value is (None, None).
-
-        .. _blue book: https://www-cdf.fnal.gov/offline/PostScript/BLUEBOOK.PDF
         """
         return self._dashes