Fix warning

Co-authored-by: pwochner <pwochner@users.noreply.github.com>
Co-authored-by: Tim Hobson <thobson@turing.ac.uk>

Author: 3 people

crates/trustchain-core/src/graph.rs

@@ -136,7 +136,7 @@ mod tests {
     fn test_display() -> Result<(), GraphError> {
         let chains = vec![test_chain(), test_chain()];
         let graph = TrustchainGraph::new(&chains, DEFAULT_LABEL_WIDTH)?;
-        format!("{}", graph);
+        let _ = format!("{}", graph);
         Ok(())
     }
 }