16405: fixed trivial doctest failure and ReST indentation

Author: nthiery

src/sage/categories/additive_magmas.py

@@ -913,10 +913,10 @@ def __neg__(self):
                         the assertion that the inputs are parents in
                         ``cartesian_product``::
 
-                        sage: cartesian_product([ZZ, NN, RR])
-                        Traceback (most recent call last):
-                        ...
-                        AssertionError
+                            sage: cartesian_product([ZZ, NN, RR])
+                            Traceback (most recent call last):
+                            ...
+                            AssertionError
                     """
                     return self.parent()(
                         -x for x in self.cartesian_factors())

src/sage/categories/magmas.py

@@ -459,7 +459,7 @@ def __invert__(self):
                         ZeroDivisionError: rational division by zero
 
                         sage: ~C([2,2,2,2])
-                        Traceback (most recent call last)
+                        Traceback (most recent call last):
                         ...
                         TypeError: no conversion of this rational to integer
                     """