Fix a slow doctest in matrix_integer_dense_hnf.py

The hnf for integer dense matrices is quite slow, reported in #35161.
While that issue is resolved, it doesn't make sense to keep this very
slow test, so tune it down to an acceptable time.

src/sage/matrix/matrix_integer_dense_hnf.py

@@ -1189,9 +1189,9 @@ def benchmark_hnf(nrange, bits=4):
     EXAMPLES::
 
         sage: import sage.matrix.matrix_integer_dense_hnf as hnf
-        sage: hnf.benchmark_hnf([50,100],32)
-        ('sage', 50, 32, ...),
-        ('sage', 100, 32, ...),
+        sage: hnf.benchmark_hnf([10,25],32)
+        ('sage', 10, 32, ...),
+        ('sage', 25, 32, ...),
     """
     b = 2**bits
     for n in nrange: