Fix errors detected by Ruff (#3918)

### What problem does this PR solve?

Fix errors detected by Ruff

### Type of change

- [x] Refactoring

rag/nlp/term_weight.py

@@ -62,10 +62,10 @@ class Dealer:
             res = {}
             f = open(fnm, "r")
             while True:
-                l = f.readline()
-                if not l:
+                line = f.readline()
+                if not line:
                     break
-                arr = l.replace("\n", "").split("\t")
+                arr = line.replace("\n", "").split("\t")
                 if len(arr) < 2:
                     res[arr[0]] = 0
                 else: