HTTP3.md: move -p for mkdir, remove -j for make

- mkdir on OSX/Darwin requires `-p` argument before dir

- portabbly figuring out number of cores is an exercise for somewhere
  else

Closes #4407

docs/HTTP3.md

@@ -91,9 +91,9 @@ Build BoringSSL (it needs to be built manually so it can be reused with curl):
      % mkdir build
      % cd build
      % cmake -DCMAKE_POSITION_INDEPENDENT_CODE=on ..
-     % make -j`nproc`
+     % make
      % cd ..
-     % mkdir .openssl/lib -p
+     % mkdir -p .openssl/lib
      % cp build/crypto/libcrypto.a build/ssl/libssl.a .openssl/lib
      % ln -s $PWD/include .openssl
 
@@ -109,7 +109,7 @@ Clone and build 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
-     % make -j`nproc`
+     % make
 
 ## Running