introduce omitted index default before using it (GH-27775)

Doc/tutorial/introduction.rst

@@ -269,14 +269,6 @@ to obtain individual characters, *slicing* allows you to obtain substring::
    >>> word[2:5]  # characters from position 2 (included) to 5 (excluded)
    'tho'
 
-Note how the start is always included, and the end always excluded.  This
-makes sure that ``s[:i] + s[i:]`` is always equal to ``s``::
-
-   >>> word[:2] + word[2:]
-   'Python'
-   >>> word[:4] + word[4:]
-   'Python'
-
 Slice indices have useful defaults; an omitted first index defaults to zero, an
 omitted second index defaults to the size of the string being sliced. ::
 
@@ -287,6 +279,14 @@ omitted second index defaults to the size of the string being sliced. ::
    >>> word[-2:]  # characters from the second-last (included) to the end
    'on'
 
+Note how the start is always included, and the end always excluded.  This
+makes sure that ``s[:i] + s[i:]`` is always equal to ``s``::
+
+   >>> word[:2] + word[2:]
+   'Python'
+   >>> word[:4] + word[4:]
+   'Python'
+
 One way to remember how slices work is to think of the indices as pointing
 *between* characters, with the left edge of the first character numbered 0.
 Then the right edge of the last character of a string of *n* characters has