Replace --traceconfig with --trace-config in doc

Since 'py.test --help' shows --trace-config as an option (and not
--traceconfig) even though both forms are accepted I assume --trace-config is
the preferred form and should therefore be used in the documentation.

doc/en/plugins.rst

@@ -90,7 +90,7 @@ Finding out which plugins are active
 If you want to find out which plugins are active in your
 environment you can type::
 
-    py.test --traceconfig
+    py.test --trace-config
 
 and will get an extended test header which shows activated plugins
 and their names. It will also print local plugins aka