Meta: Fix error in lint-ports.py

The data structures here were changed from sets to dicts at some point,
with the sets now having different names - this would crash trying to
subtract two dicts.

Meta/lint-ports.py

@@ -193,7 +193,7 @@ def run():
     if from_table_set - ports_set:
         all_good = False
         print('AvailablePorts.md lists ports that do not appear in the file system:')
-        for port in sorted(from_table - ports):
+        for port in sorted(from_table_set - ports_set):
             print(f"    {port}")
 
     if ports_set - from_table_set: