Fix typo: swap node and match in the example (#1997)

pep-0635.rst

@@ -1148,7 +1148,7 @@ Using modern syntax, a depth-first tree traversal would then be written as
 follows::
 
   def traverse(node):
-      node match:
+      match node:
           case Node(left, right):
               traverse(left)
               traverse(right)