[mdavis-xyz] #89 Fix typos in slicing tutorial

Fixes #89

documentation/tutorial.rst

@@ -100,7 +100,7 @@ we can use this expression:
 
     [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
 
-The two example above can be shortened.  If the ``start`` or ``step`` value is
+The two example above can be shortened.  If the ``start`` or ``stop`` value is
 omitted it is assumed to be the start or the end of the array.  For example:
 
 .. jpexample:: [:5]
@@ -109,7 +109,7 @@ omitted it is assumed to be the start or the end of the array.  For example:
 
     [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
 
-Try modifying changing the example above to only include the last half of the
+Try modifying the example above to only include the last half of the
 array elements without specifying the end value of ``10``.
 
 The general form of a slice is ``[start:stop:step]``.  So far we've looked