Fix UB in heap init example docs

[kurtjd]

_heap_size is not actually a valid address anywhere (of course we have to treat it as an address since it is a linker symbol), and therefore creating a reference to it, even as an intermediate step, is undefined behavior. It seems in debug mode rust will panic if we create a reference to null (e.g. if _heap_size is zero) even though we aren't attempting to access it.

This is just quick fix to replace that line with addr_of! which doesn't create an intermediate reference and therefore no longer UB. The macro is apparently now soft deprecated and suggests replacement with &raw const directly, but kept the macro for consistency with its other uses in code base.