fixes small residual from copy-paste in call_material_from_elabid

the fix changes "sample_data" - which was residual from copy-pasting the
call_entrypoint... function and therefore undefined - with
"material_data" which is very well defined in the previous line

src/main.py

@@ -29,7 +29,7 @@ def call_material_from_elabid(elabid):
     try: 
         material_data = APIHandler(apikey).get_entry_from_elabid(elabid, entryType="items")
         # TO-DO: correct this typo on elabftw: Subtrate → Substrate.
-        if not sample_data.get("category_title") in ["PLD Target", "Substrate batch", "Subtrate batch"]:
+        if not material_data.get("category_title") in ["PLD Target", "Substrate batch", "Subtrate batch"]:
             raise ValueError(f"The referenced resource (elabid = {elabid}) is not a material.")
         material_object = Material(material_data)
     except ConnectionError as e: