samples: shell_module: fix build on 64bit platforms

After the __printf_like validation was added to the shell_fprintf
functions we got warnings while building the
'samples/subsys/shell/shell_module' due to incorrect specifiers
while printing size_t variables. Fix that.

Fixes: #39386
Fixes: fe2075d1 "shell: Add __printf_like to shell_fprintf"

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>