Update implementations.tex (#482)

* Update implementations.tex

In the suggested implementation which uses memory map, clarify that the region should only be accessible in Debug Mode.

If this region is accessible in non-debug Mode, it is unclear how the Debug MOdule is supposed to interpret the interactions with the memory regions specified here.

* Update implementations.tex

core -> hart for consistency

* Update implementations.tex

Pluralize "harts" as several harts may be in debug mode at once.