Fix the error name

Signed-off-by: Paweł Renc <prenc@mgh.harvard.edu>

src/MEDS_transforms/extract/merge_to_MEDS_cohort.py

@@ -69,7 +69,7 @@ def merge_subdirs_and_sort(
         ...     merge_subdirs_and_sort(sp_dir, event_subsets=[], unique_by=None)
         Traceback (most recent call last):
             ...
-        FileNotFoundError: No files found in ...
+        FileNotFoundError: No parquet files found in ...
         >>> with TemporaryDirectory() as tmpdir:
         ...     sp_dir = Path(tmpdir)
         ...     (sp_dir / "subdir1").mkdir()