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

doc/en/test/attic.rst

@@ -21,7 +21,7 @@ but note that project specific settings will be considered
 first.  There is a flag that helps you debugging your
 conftest.py configurations::
 
-    py.test --traceconfig
+    py.test --trace-config
 
 
 customizing the collecting and running process

doc/en/test/plugin/helpconfig.rst

@@ -16,7 +16,7 @@ command line options
     display py lib version and import information.
 ``-p name``
     early-load given plugin (multi-allowed).
-``--traceconfig``
+``--trace-config``
     trace considerations of conftest.py files.
 ``--nomagic``
     don't reinterpret asserts, no traceback cutting.

doc/en/writing_plugins.rst

@@ -194,7 +194,7 @@ the plugin manager like this:
     plugin = config.pluginmanager.getplugin("name_of_plugin")
 
 If you want to look at the names of existing plugins, use
-the ``--traceconfig`` option.
+the ``--trace-config`` option.
 
 Testing plugins
 ---------------