[Doc] Add doctest directive to an example in stdtypes

Co-authored-by: Erlend E. Aasland <[email protected]>

Doc/library/stdtypes.rst

@@ -2333,8 +2333,10 @@ String objects have one unique built-in operation: the ``%`` operator (modulo).
 This is also known as the string *formatting* or *interpolation* operator.
 Given ``format % values`` (where *format* is a string), ``%`` conversion
 specifications in *format* are replaced with zero or more elements of *values*.
-The effect is similar to using the :c:func:`sprintf` in the C language. For
-example::
+The effect is similar to using the :c:func:`sprintf` function in the C language.
+For example:
+
+.. doctest::
 
    >>> print('%s has %d quote types.' % ('Python', 2))
    Python has 2 quote types.