gh-106318: Add doctest role for str.capitalize() examples

Author: adorilson

Doc/library/stdtypes.rst

@@ -1759,7 +1759,9 @@ expression support in the :mod:`re` module).
 .. method:: str.capitalize()
 
    Return a copy of the string with its first character capitalized and the
-   rest lowercased. For example::
+   rest lowercased. For example:
+
+   .. doctest::
 
       >>> 'PYTHON IS AMAZING'.capitalize()
       'Python is amazing'
@@ -1773,6 +1775,7 @@ expression support in the :mod:`re` module).
       This means that characters like digraphs will only have their first
       letter capitalized, instead of the full character.
 
+
 .. method:: str.casefold()
 
    Return a casefolded copy of the string. Casefolded strings may be used for