Fixed "unknown command-line argument" output format.

Change-Id: Idd49bc61118c29d6d7364d2cd54a6f7932917570

src/flatc.cpp

@@ -243,7 +243,7 @@ int main(int argc, const char *argv[]) {
             goto found;
           }
         }
-        Error("unknown commandline argument" + arg, true);
+        Error("unknown commandline argument: " + arg, true);
         found:;
       }
     } else {