Fix regression in unique symbol type generation (#1096)

* fix unique symbol type.

Originally we'd write `symbol` here. TypeDoc changed and we started
emitting 'TODO'.

Author: AndrewJakubowicz

packages/lit-dev-content/site/_includes/api.html

@@ -61,7 +61,7 @@
     {%- elif t.operator == 'keyof' -%}
       keyof {{ type(t.target) }}
 
-    {%- elif t.operator == 'unique' and t.target.name == 'symbol' -%}
+    {%- elif t.type == "typeOperator" and t.operator == "unique" -%}
       symbol
 
     {%- elif t.type == 'conditional' -%}