docs/HTTP3: add --enable-alt-svc to curl's configure

docs/HTTP3.md

@@ -65,7 +65,7 @@ Build curl
      % git clone https://github.com/curl/curl
      % cd curl
      % ./buildconf
-     % LDFLAGS="-Wl,-rpath,<somewhere1>/lib" ./configure --with-ssl=<somewhere1> --with-nghttp3=<somewhere2> --with-ngtcp2=<somewhere3>
+     % LDFLAGS="-Wl,-rpath,<somewhere1>/lib" ./configure --with-ssl=<somewhere1> --with-nghttp3=<somewhere2> --with-ngtcp2=<somewhere3> --enable-alt-svc
      % make
 
 # quiche version
@@ -93,13 +93,13 @@ Build quiche:
      % cd ../..
      % QUICHE_BSSL_PATH=$PWD/deps/boringssl cargo build --release --features pkg-config-meta
 
-Clone and build curl:
+Build curl:
 
      % cd ..
      % git clone https://github.com/curl/curl
      % cd curl
      % ./buildconf
-     % ./configure LDFLAGS="-Wl,-rpath,$PWD/../quiche/target/release" --with-ssl=$PWD/../quiche/deps/boringssl/.openssl --with-quiche=$PWD/../quiche/target/release
+     % ./configure LDFLAGS="-Wl,-rpath,$PWD/../quiche/target/release" --with-ssl=$PWD/../quiche/deps/boringssl/.openssl --with-quiche=$PWD/../quiche/target/release --enable-alt-svc
      % make
 
 ## Run