Docs: fix python tutorials (#1527)

Fix mistakes reported by @Helveg in the Python tutorial docs. 

doc/tutorial/single_cell_model.rst

@@ -48,7 +48,7 @@ Our *single-segment HH cell* has a simple morphology and dynamics, constructed a
     labels = arbor.label_dict({'soma':   '(tag 1)',
                                'midpoint': '(location 0 0.5)'})
 
-    # (3) Create cell and set properties
+    # (3) Create and set up a decor object
     decor = arbor.decor()
     decor.set_property(Vm=-40)
     decor.paint('"soma"', 'hh')

doc/tutorial/single_cell_recipe.rst

@@ -93,6 +93,10 @@ It returns `0` by default and models without cells are quite boring!
             # (4.7) Override the global_properties method
             return self.the_props
 
+    # (5) Instantiate recipe with a voltage probe located on "midpoint".
+
+    recipe = single_recipe(cell, [arbor.cable_probe_membrane_voltage('"midpoint"')])
+
 Step **(4)** describes the recipe that will reflect our single cell model.
 
 Step **(4.1)** defines the class constructor. It can take any shape you need, but it

python/example/single_cell_model.py

@@ -11,7 +11,7 @@ tree.append(arbor.mnpos, arbor.mpoint(-3, 0, 0, 3), arbor.mpoint(3, 0, 0, 3), ta
 labels = arbor.label_dict({'soma':   '(tag 1)',
                            'midpoint': '(location 0 0.5)'})
 
-# (3) Create cell and set properties
+# (3) Create and set up a decor object
 decor = arbor.decor()
 decor.set_property(Vm=-40)
 decor.paint('"soma"', 'hh')