Update preprocessor_config.json

Hello

@Xenova
,
Thank you for the good work, having an ONNX version of this model is quite nice! I found out that only specifying the longest edge does not work with latest version of `transformers` (4.38.1), and using either `DetrImageProcessor`, `DetrFeatureExtractor` (deprecated) or `AutoImageProcessor` classes for instantiation. Rolling back to the configuration from `Xenova/table-transformer-structure-recognition` does seem to give good results, i.e.:
```
"size": {
"longest_edge": 1000,
"shortest_edge": 800
}
```
What do you think?

preprocessor_config.json

@@ -18,6 +18,7 @@
   "resample": 2,
   "rescale_factor": 0.00392156862745098,
   "size": {
-    "longest_edge": 800
+    "longest_edge": 1000,
+    "shortest_edge": 800
   }
 }