utils: Fix metadata attributes.

Should be `name` not `key`.

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>

v2x/lib/mux.py

diff --git a/v2x/lib/mux.py b/v2x/lib/mux.py
index 51122a1..ca31ee2 100644
--- a/v2x/lib/mux.py
+++ b/v2x/lib/mux.py
@@ -29,9 +29,9 @@
 
 def add_metadata(tag, mtype, msubtype):
     meta_root = ET.SubElement(tag, 'metadata')
-    meta_type = ET.SubElement(meta_root, 'meta', {'key': 'type'})
+    meta_type = ET.SubElement(meta_root, 'meta', {'name': 'type'})
     meta_type.text = mtype
-    meta_subtype = ET.SubElement(meta_root, 'meta', {'key': 'subtype'})
+    meta_subtype = ET.SubElement(meta_root, 'meta', {'name': 'subtype'})
     meta_subtype.text = msubtype