doc: fix a typo in unittest.mock.rst (GH-30227)

(cherry picked from commit 10bf0a9ac3c9bbde2fa8989d9435462f0e7da545)

Co-authored-by: Joe <nigelchiang@outlook.com>
diff --git a/Doc/library/unittest.mock.rst b/Doc/library/unittest.mock.rst
index d0640b4..4d74f9c 100644
--- a/Doc/library/unittest.mock.rst
+++ b/Doc/library/unittest.mock.rst
@@ -1516,7 +1516,7 @@
     >>> test()
     Traceback (most recent call last):
       ...
-    AttributeError: <module 'sys' (built-in)> does not have the attribute 'non_existing'
+    AttributeError: <module 'sys' (built-in)> does not have the attribute 'non_existing_attribute'
 
 but adding ``create=True`` in the call to :func:`patch` will make the previous example
 work as expected::