Fix bug in hierarchical_merge function (#4006)

### What problem does this PR solve?

Fix hierarchical_merge function. From idx vs. actual value to actual
value vs. actual value.
Related issue #4003

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: luopan <[email protected]>

rag/nlp/__init__.py

@@ -466,7 +466,7 @@ def hierarchical_merge(bull, sections, depth):
                 jj = binary_search(levels[ii], j)
                 if jj < 0:
                     continue
-                if jj > cks[-1][-1]:
+                if levels[ii][jj] > cks[-1][-1]:
                     cks[-1].pop(-1)
                 cks[-1].append(levels[ii][jj])
             for ii in cks[-1]: