fixed spelling of mingw-3_4_2 in iostreams

[SVN r29326]

status/explicit-failures-markup.xml

@@ -899,7 +899,7 @@
             <test name="wide_stream_test"/>
             <toolset name="gcc-2.95.3-linux"/>
             <toolset name="mingw"/>
-            <toolset name="mingw-3.4.2"/>
+            <toolset name="mingw-3_4_2"/>
             <note author="Jonathan Turkanis">
                 No wide stream support.
             </note>