net: shell: quic: Fix -Wformat warning on 64-bit platforms

Use the PRIu64 macro from inttypes.h to format the stream ID,
which is of type uint64_t. This change resolves the -Wformat
warning on 64-bit platforms:

format '%llu' expects argument of type 'long long unsigned int',
but argument 6 has type 'uint64_t' {aka 'long unsigned int'}

Signed-off-by: Tim Pambor <tim.pambor@codewrights.de>