Update preprocessor_config.json

Training the efficientformer is failed because of this shortest_edge option. The shortest_edge must be 224, not 256.

preprocessor_config.json

@@ -21,6 +21,6 @@
   "resample": 3,
   "rescale_factor": 0.00392156862745098,
   "size": {
-    "shortest_edge": 256
+    "shortest_edge": 224
   }
 }