DOC: Fix grammar in dMRI model comment

Fix grammar in dMRI model comment.

Author: jhlegarreta

src/nifreeze/model/dmri.py

@@ -67,7 +67,7 @@ def __init__(self, dataset: DWI, max_b: float | int | None = None, **kwargs):
 
         """
 
-        # Duck typing, instead of explicitly test for DWI type
+        # Duck typing, instead of explicitly testing for DWI type
         if not hasattr(dataset, "bzero"):
             raise TypeError("Dataset MUST be a DWI object.")