Fix dataset download url

Thank you for your wonderful work in creating and sharing this dataset!

However, when I was trying to load the dataset according to the instruction given in README, I got: `... FileNotFoundError: Unable to resolve any data file that matches ['**'] in dataset repository csebuetnlp/CrossSum with any supported extension ...` and `AttributeError: 'NoneType' object has no attribute 'version_str'`.

The former error can be solved by loading the dataset in lower case, namely: `load_dataset(f"csebuetnlp/crosssum", "{}-{}".format(src_lang, tgt_lang))`;

This pull request is opened to fix the latter error by using the correct format for dataset download url.

crosssum.py

@@ -116,7 +116,9 @@ class Crosssum(datasets.GeneratorBasedBuilder):
     def _split_generators(self, dl_manager):
         """Returns SplitGenerators."""
         lang = str(self.config.name)
-        url = _URL.format(lang, self.VERSION.version_str[:-2])
+        src_lang, tgt_lang = lang.split("-")
+        # url = _URL.format(lang, self.VERSION.version_str[:-2])
+        url = _URL.format(src_lang, tgt_lang)
 
         data_dir = dl_manager.download_and_extract(url)
         return [