Change RISC-V OpenOCD to Official Fork in README

[a-gavin]

As of this PR the official RISC-V OpenOCD fork supports the Longan Nano. I personally have had trouble both running and building the currently linked unofficial RISC-V OpenOCD fork in the README on all tagged versions, i.e. current version segfaults in my setup, am unable to build previous versions without compiler errors.

While the unofficial fork is still maintained, given that the official RISC-V OpenOCD fork supports this board, I suggest pointing users to the official fork instead.