SF bug #699237: Tutorial uses omitted slice indices before explaining them

Moved up the explanation of slice default arguments.

Doc/tut/tut.tex

@@ -640,6 +640,17 @@ separated by a colon.
 'lp'
 \end{verbatim}
 
+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.
+
+\begin{verbatim}
+>>> word[:2]    # The first two characters
+'He'
+>>> word[2:]    # All but the first two characters
+'lpA'
+\end{verbatim}
+
 Unlike a C string, Python strings cannot be changed.  Assigning to an 
 indexed position in the string results in an error:
 
@@ -664,17 +675,6 @@ efficient:
 'SplatA'
 \end{verbatim}
 
-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.
-
-\begin{verbatim}
->>> word[:2]    # The first two characters
-'He'
->>> word[2:]    # All but the first two characters
-'lpA'
-\end{verbatim}
-
 Here's a useful invariant of slice operations:
 \code{s[:i] + s[i:]} equals \code{s}.