Fix typo.

Author: djunglas

pyomo/solvers/tests/checks/test_xpress_persistent.py

@@ -535,7 +535,7 @@ def test_callbacks_01(self):
         """Simple callback test.
 
         Tests that optnode, preintsol, intsol callbacks are invoked.
-        Also tests that information between preintsol an intsol callbacks
+        Also tests that information between preintsol and intsol callbacks
         is consistent.
         """
         model = self._markshare()