Update argmin docs to reflect the code behavior (pytorch#78888)

Fixes pytorch#78791 Pull Request resolved: pytorch#78888 Approved by: https://github.com/ezyang

Author: andrijdavid

torch/_torch_docs.py

@@ -6731,7 +6731,7 @@ def merge_dicts(*dicts):
 Args:
  {input}
  {dim} If ``None``, the argmin of the flattened input is returned.
- {keepdim} Ignored if ``dim=None``.
+ {keepdim}.
 
 Example::