update docstring

src/nplinker/metabolomics/gnps/gnps_molecular_family_loader.py

@@ -20,7 +20,7 @@ def __init__(self, file: str | PathLike):
 
         The "ComponentIndex" column in the GNPS molecular family's file is treated
         as family id. But for molecular families that have only one member (i.e. spectrum),
-        named singleton molecular families, have the same value of
+        named singleton molecular families, their files have the same value of
         "-1" in the "ComponentIndex" column. To make the family id unique,the
         spectrum id plus a prefix "singleton-" is used as the family id of
         singleton molecular families.