Fix number of samples in README

The number of samples in each split is incorrect in the README. This results in the following error when loading the dataset:
```
datasets.utils.info_utils.NonMatchingSplitsSizesError: [{'expected': SplitInfo(name='train', num_bytes=0, num_examples=3834, shard_lengths=None, dataset_name=None), 'recorded': SplitInfo(name='train', num_bytes=239557502, num_examples=3835, shard_lengths=None, dataset_name='popp-line')}, {'expected': SplitInfo(name='validation', num_bytes=0, num_examples=479, shard_lengths=None, dataset_name=None), 'recorded': SplitInfo(name='validation', num_bytes=29370544, num_examples=480, shard_lengths=None, dataset_name='popp-line')}, {'expected': SplitInfo(name='test', num_bytes=0, num_examples=478, shard_lengths=None, dataset_name=None), 'recorded': SplitInfo(name='test', num_bytes=29663264, num_examples=479, shard_lengths=None, dataset_name='popp-line')}]
```

README.md

@@ -13,12 +13,12 @@ dataset_info:
     dtype: string
   splits:
   - name: train
-    num_examples: 3834
+    num_examples: 3835
   - name: validation
-    num_examples: 479
+    num_examples: 480
   - name: test
-    num_examples: 478
-  dataset_size: 4791
+    num_examples: 479
+  dataset_size: 4794
 tags:
 - atr
 - htr