Update src/io/rsm/raft.hpp

Co-authored-by: János Benjamin Antal <antaljanosbenjamin@users.noreply.github.com>

src/io/rsm/raft.hpp

@@ -354,7 +354,7 @@ class Raft {
       return 0;
     }
 
-    auto &[term, data] = state_.log.at(index - 1);
+    const auto &[term, data] = state_.log.at(index - 1);
     return term;
   }