Fix markdown

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #24037)

demos/quic/server/README.md

@@ -8,12 +8,12 @@ Type `make` to build and `make run` to run.
 
 Usage:
 
-```
+```bash
 ./server <port-number> <certificate-file> <key-file>
 ```
 
 Example client usage:
 
-```
+```bash
 openssl s_client -quic -alpn ossltest -connect 127.0.0.1:<port-number>
 ```