HTTP3.md: clarify cargo build directory

Cargo needs to be called from within the 'quiche' directory.

Closes #5522

docs/HTTP3.md

@@ -115,6 +115,7 @@ Build curl
 Build quiche and BoringSSL:
 
      % git clone --recursive https://github.com/cloudflare/quiche
+     % cd quiche
      % cargo build --release --features pkg-config-meta,qlog
      % mkdir deps/boringssl/lib
      % ln -vnf $(find target/release -name libcrypto.a -o -name libssl.a) deps/boringssl/lib/