DOC: Add missing :ref: to a link in a docstring (#50012)

Author: jnclt

pandas/core/frame.py

@@ -5168,7 +5168,7 @@ def drop(
         Remove rows or columns by specifying label names and corresponding
         axis, or by specifying directly index or column names. When using a
         multi-index, labels on different levels can be removed by specifying
-        the level. See the `user guide <advanced.shown_levels>`
+        the level. See the :ref:`user guide <advanced.shown_levels>`
         for more information about the now unused levels.
 
         Parameters