bpo-33952: Fix typo in str.upper() documentation (GH-7898) (GH-7903)

(cherry picked from commit 4a6e746079441d18c30e3c4d014f106faaf7792f)

Co-authored-by: Andrés Delfino <adelfino@gmail.com>
diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
index 7cab670..03f9159 100644
--- a/Doc/library/stdtypes.rst
+++ b/Doc/library/stdtypes.rst
@@ -2051,7 +2051,7 @@
 .. method:: str.upper()
 
    Return a copy of the string with all the cased characters [4]_ converted to
-   uppercase.  Note that ``str.upper().isupper()`` might be ``False`` if ``s``
+   uppercase.  Note that ``s.upper().isupper()`` might be ``False`` if ``s``
    contains uncased characters or if the Unicode category of the resulting
    character(s) is not "Lu" (Letter, uppercase), but e.g. "Lt" (Letter,
    titlecase).