fix one easy doctest

Author: fchapoton

src/sage/categories/commutative_rings.py

@@ -589,7 +589,7 @@ def _pseudo_fraction_field(self):
                 sage: Integers(15).fraction_field()
                 Traceback (most recent call last):
                 ...
-                TypeError: self must be an integral domain.
+                TypeError: self must be an integral domain
 
             TESTS::