Update postgres_impl.py

There is aBUG. When lighttrag uses the postgres@15 version, there will be an error.The BUG detail as follow:
PostgreSQL database, error:subquery in FROM must have an alias
HINT:  For example, FROM (SELECT ...) [AS] foo.
It roughly means that there is an error in SQL template, and you need to use the alias alias to declare the content of the subquery.

lightrag/kg/postgres_impl.py

@@ -1739,7 +1739,7 @@ SQL_TEMPLATES = {
                 FROM LIGHTRAG_VDB_ENTITY
                 where workspace=$1
                 AND chunk_id IN (SELECT chunk_id FROM relevant_chunks)
-            )
+            ) as chunk_distances
         WHERE distance>$2
         ORDER BY distance DESC
         LIMIT $3
@@ -1756,7 +1756,7 @@ SQL_TEMPLATES = {
                 FROM LIGHTRAG_DOC_CHUNKS
                 where workspace=$1
                 AND id IN (SELECT chunk_id FROM relevant_chunks)
-            )
+            ) as chunk_distances
             WHERE distance>$2
             ORDER BY distance DESC
             LIMIT $3