HTTP3: show an --alt-svc using example too

docs/HTTP3.md

@@ -75,8 +75,14 @@ you'll just get ld.so linker errors.
 
 ## Invoke from command line
 
+Use HTTP/3 directly:
+
     curl --http3 https://nghttp2.org:8443/
 
+Upgrade via Alt-Svc:
+
+    curl --alt-svc altsvc.cache https://quic.aiortc.org/
+
 # quiche version
 
 ## build