HTTP3.md: always run nghttp3 submodule init

- For consistency change all 'build nghttp3' commands to run submodule
  init after cloning, even if the branch does not have submodules.

Follow-up to 5a4b2f93 and 4f794558.

Closes https://github.com/curl/curl/pull/12928

docs/HTTP3.md

@@ -102,6 +102,7 @@ Build nghttp3
      % cd ..
      % git clone -b v1.1.0 https://github.com/ngtcp2/nghttp3
      % cd nghttp3
+     % git submodule update --init
      % autoreconf -fi
      % ./configure --prefix=<somewhere2> --enable-lib-only
      % make
@@ -143,6 +144,7 @@ Build nghttp3
      % cd ..
      % git clone -b v1.1.0 https://github.com/ngtcp2/nghttp3
      % cd nghttp3
+     % git submodule update --init
      % autoreconf -fi
      % ./configure --prefix=<somewhere2> --enable-lib-only
      % make