fix setenv prepend docs (#5658)

fix setenv prepend docs

doc/en/monkeypatch.rst

@@ -46,7 +46,7 @@ environment variable is missing, or to set multiple values to a known variable.
 :py:meth:`monkeypatch.setenv` and :py:meth:`monkeypatch.delenv` can be used for
 these patches.
 
-4. Use ``monkeypatch.setenv("PATH", value, prepend=True)`` to modify ``$PATH``, and
+4. Use ``monkeypatch.setenv("PATH", value, prepend=os.pathsep)`` to modify ``$PATH``, and
 :py:meth:`monkeypatch.chdir` to change the context of the current working directory
 during a test.