DOC: Fix the string example. (#30762)

Author: TomAugspurger

doc/source/user_guide/text.rst

@@ -87,8 +87,9 @@ l. For ``StringDtype``, :ref:`string accessor methods<api.series.str>`
 
    .. ipython:: python
 
-      s.astype(object).str.count("a")
-      s.astype(object).dropna().str.count("a")
+      s2 = pd.Series(["a", None, "b"], dtype="object")
+      s2.str.count("a")
+      s2.dropna().str.count("a")
 
    When NA values are present, the output dtype is float64. Similarly for
    methods returning boolean values.