Fix missing references in doc build. (matplotlib#15134)

Author: anntzer

doc/api/next_api_changes/2018-02-18-AL.rst

@@ -1,9 +1,9 @@
 Deprecations
 ````````````
 
-For the `mpl_toolkits.axistartist.axis_artist.AttributeCopier` class, the
+For the ``mpl_toolkits.axistartist.axis_artist.AttributeCopier`` class, the
 constructor and the ``set_ref_artist`` method, and the *default_value*
 parameter of ``get_attribute_from_ref_artist``, are deprecated.
 
 Deprecation of the constructor means that classes inheriting from
-`AttributeCopier` should no longer call its constructor.
+``AttributeCopier`` should no longer call its constructor.