sparse_advection format string incorrect

[BenWibking]

There's a minor format string error in the sparse_advection example:

[146/154] Building CXX object example/sparse_advection/CMakeFiles/sparse_advection-example.dir/parthenon_app_inputs.cpp.o
/Users/benwibking/parthenon/example/sparse_advection/parthenon_app_inputs.cpp:180:17: warning: format specifies type 'long' but the argument has type 'std::uint64_t' (aka 'unsigned long long') [-Wformat]
                mem_min, mem_max, mem_avg);
                ^~~~~~~
/Users/benwibking/parthenon/example/sparse_advection/parthenon_app_inputs.cpp:180:26: warning: format specifies type 'long' but the argument has type 'std::uint64_t' (aka 'unsigned long long') [-Wformat]
                mem_min, mem_max, mem_avg);
                         ^~~~~~~
2 warnings generated.