Fix markup glitch in unittest docs. Will backport.

Doc/lib/libunittest.tex

@@ -226,7 +226,7 @@ runs, an exception will be raised, and the testing framework will
 identify the test case as a \dfn{failure}.  Other exceptions that do
 not arise from checks made through the \method{assert*()} and
 \method{fail*()} methods are identified by the testing framework as
-dfn{errors}.
+\dfn{errors}.
 
 The way to run a test case will be described later.  For now, note
 that to construct an instance of such a test case, we call its