fix deprecated warning

bellman/src/cs.rs

@@ -199,7 +199,7 @@ impl fmt::Display for SynthesisError {
             write!(f, "I/O error: ")?;
             e.fmt(f)
         } else {
-            write!(f, "{}", self.description())
+            write!(f, "{}", self.to_string())
         }
     }
 }

pairing/src/lib.rs

@@ -310,7 +310,7 @@ impl fmt::Display for GroupDecodingError {
             GroupDecodingError::CoordinateDecodingError(description, ref err) => {
                 write!(f, "{} decoding error: {}", description, err)
             }
-            _ => write!(f, "{}", self.description()),
+            _ => write!(f, "{}", self.to_string()),
         }
     }
 }