Fix compilation warning when compiling with -Wformat flag

[PBrunot]

Compiler warning : format '%lu' expects argument of type 'long unsigned int', but argument 3 has type 'uint32_t'

The compiler warning arises because the format specifier %lu expects a long unsigned int, but the variable free is of type uint32_t (which is typically unsigned int). The ESP32, part of the Espressif32 family, typically defines uint32_t as unsigned int, so using the %u format specifier is appropriate.