Fix thinko in a comment about seeking with a file object.

Reported by Francesco Trentini.

Doc/tut/tut.tex

@@ -2873,7 +2873,7 @@ using the beginning of the file as the reference point.
 \begin{verbatim}
 >>> f=open('/tmp/workfile', 'r+')
 >>> f.write('0123456789abcdef')
->>> f.seek(5)     # Go to the 5th byte in the file
+>>> f.seek(5)     # Go to the 6th byte in the file
 >>> f.read(1)        
 '5'
 >>> f.seek(-3, 2) # Go to the 3rd byte before the end