Fix MarlinPC's CommitterKey to return the correct supported_degree (#78)

CHANGELOG.md

 ## Pending
+- MarlinPC's `supported_degree` fix.
 
 ### Breaking changes
 

src/marlin/marlin_pc/data_structures.rs

@@ -88,7 +88,7 @@ impl<E: PairingEngine> PCCommitterKey for CommitterKey<E> {
     }
 
     fn supported_degree(&self) -> usize {
-        self.powers.len()
+        self.powers.len() - 1
     }
 }