add series.str.casefold in reference

Author: charlesdong1991

doc/source/reference/series.rst

@@ -409,6 +409,7 @@ strings and apply several methods to it. These can be accessed like
    :template: autosummary/accessor_method.rst
 
    Series.str.capitalize
+   Series.str.casefold
    Series.str.cat
    Series.str.center
    Series.str.contains

pandas/tests/test_strings.py

@@ -3426,8 +3426,8 @@ def test_method_on_bytes(self):
             tm.assert_series_equal(result, expected)
 
     def test_casefold(self):
-        casefolded = Series(['ss', NA, 'case', 'ssd'])
-        s = Series(['ß', NA, 'case', 'ßd'])
+        casefolded = Series(['ss', 'case', 'ssd'])
+        s = Series(['ß', 'case', 'ßd'])
         result = s.str.casefold()
 
         tm.assert_series_equal(result, casefolded)