Update README.md

The original code sample would throw this error ```TypeError: unsupported operand type(s) for +: 'Tensor' and 'NoneType'```.
Per https://github.com/mlfoundations/open_clip/issues/736#issuecomment-1801879013, the updated sample will run without issue.

README.md

@@ -91,7 +91,8 @@ with torch.no_grad(), torch.cuda.amp.autocast():
     image_features = F.normalize(image_features, dim=-1)
     text_features = F.normalize(text_features, dim=-1)
 
-    text_probs = torch.sigmoid(image_features @ text_features.T * model.logit_scale.exp() + model.logit_bias)
+    logits = model.logit_scale.exp() * image_features @ text_features.T
+    probs = logits.softmax(dim=-1)
 
 zipped_list = list(zip(labels_list, [round(p.item(), 3) for p in text_probs[0]]))
 print("Label probabilities: ", zipped_list)