doc(manpage): add missing --conclude header

doc/hyperfine.1

@@ -117,6 +117,7 @@ The \fB\-\-prepare\fR option can be specified once for all commands or multiple
 once for each command. In the latter case, each preparation command will be
 run prior to the corresponding benchmark command.
 .HP
+\fB\-\-conclude\fR \fICMD...\fP
 .IP
 Execute \fICMD\fP after each timing run. This is useful for clearing disk caches,
 for example.