dirstate: remove obsolete reference to dirstate.beginparentchange

The only valid API since 265e91da56fd (dirstate: drop deprecated
methods (API), 2018-02-02) is the context manager returned from
dirstate.parentchange().

Differential Revision: https://phab.mercurial-scm.org/D6126

mercurial/dirstate.py

@@ -289,8 +289,8 b' class dirstate(object):'
         See localrepo.setparents()
         """
         if self._parentwriters == 0:
-            raise ValueError("cannot set dirstate parent without "
-                             "calling dirstate.beginparentchange")
+            raise ValueError("cannot set dirstate parent outside of "
+                             "dirstate.parentchange context manager")
 
         self._dirty = True
         oldp2 = self._pl[1]