gh-106318: Fix incorrectly rendered code block in str.isalnum() docs (GH-144718)

Doc/library/stdtypes.rst

@@ -2183,7 +2183,7 @@ expression support in the :mod:`re` module).
    Return ``True`` if all characters in the string are alphanumeric and there is at
    least one character, ``False`` otherwise.  A character ``c`` is alphanumeric if one
    of the following returns ``True``: ``c.isalpha()``, ``c.isdecimal()``,
-   ``c.isdigit()``, or ``c.isnumeric()``. For example::
+   ``c.isdigit()``, or ``c.isnumeric()``. For example:
 
    .. doctest::