[Doc]: Update random_sampler.py (#9033)

neg_pos_up -> neg_pos_ub
minor typo fix

Author: Ryoo72

mmdet/core/bbox/samplers/random_sampler.py

@@ -12,7 +12,7 @@ class RandomSampler(BaseSampler):
     Args:
         num (int): Number of samples
         pos_fraction (float): Fraction of positive samples
-        neg_pos_up (int, optional): Upper bound number of negative and
+        neg_pos_ub (int, optional): Upper bound number of negative and
             positive samples. Defaults to -1.
         add_gt_as_proposals (bool, optional): Whether to add ground truth
             boxes as proposals. Defaults to True.