DOC: Mark the insert_b0 parameter as optional

Mark the `insert_b0` parameter as optional.

Author: jhlegarreta

src/nifreeze/data/dmri.py

@@ -216,7 +216,7 @@ def to_nifti(self, filename: Path | str, insert_b0: bool = False) -> None:
         ----------
         filename : :obj:`os.pathlike`
             The output NIfTI file path.
-        insert_b0 : :obj:`bool`
+        insert_b0 : :obj:`bool`, optional
             Insert a :math:`b=0` at the front of the output NIfTI.
 
         """