chore: update operators.py (#2724)

### What problem does this PR solve?
substract -> subtract
### Type of change

- [ ] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [x] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

deepdoc/vision/operators.py

@@ -102,7 +102,7 @@ class StandardizeImage(object):
 
 
 class NormalizeImage(object):
-    """ normalize image such as substract mean, divide std
+    """ normalize image such as subtract mean, divide std
     """
 
     def __init__(self, scale=None, mean=None, std=None, order='chw', **kwargs):