doc: remove non-existing command riscv set_scratch_ram

doc: remove non-existing command riscv set_scratch_ram

The command was proposed in http://review.openocd.org/#/c/4578/2
and dropped in favor on existing work area commands.
Nevertheless the command landed in documentation.

Remove it!

Change-Id: I6e18124256f29be15d8593c07d96f61d19e983f8
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: a51ab8ddf63a ("Add RISC-V support.")
Reviewed-on: https://review.openocd.org/c/openocd/+/6721
Tested-by: jenkins
Reviewed-by: Jan Matyas <matyas@codasip.com>