[Fix] Fix a grammatical error in docstr (open-mmlab#8639)

Author: gaotongxiao

Diff for: mmdet/models/detectors/base.py

@@ -69,8 +69,8 @@ def forward(self,
         - "loss": Forward and return a dict of losses according to the given
         inputs and data samples.
 
-        Note that this method doesn't handle neither back propagation nor
-        optimizer updating, which are done in the :meth:`train_step`.
+        Note that this method doesn't handle either back propagation or
+        parameter update, which are supposed to be done in :meth:`train_step`.
 
         Args:
             inputs (torch.Tensor): The input tensor with shape