Update lib/matplotlib/_api/deprecation.py

Co-authored-by: Ruth Comer <[email protected]>

Author: timhoffm

lib/matplotlib/_api/deprecation.py

@@ -432,7 +432,7 @@ def violinplot(self, dataset, positions=None, vert=None, ...)
         @_api.make_keyword_only("3.10", "vert")
         def violinplot(self, dataset, positions=None, vert=None, ...)
 
-    When the deprecation expires, switch to
+    When the deprecation expires, switch to ::
 
         def violinplot(self, dataset, positions=None, *, vert=None, ...)