board/spike/riscv64/readme.txt: update riscv-openocd URL

The URL https://github.com/riscv/riscv-openocd is now redirecting to:
https://github.com/riscv-collab/riscv-openocd

This commit updates the URL to this new location.

Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/spike/riscv64/readme.txt

@@ -28,4 +28,4 @@ https://github.com/riscv-software-src/riscv-isa-sim/tree/v1.1.0#simulating-a-new
 https://github.com/riscv-software-src/riscv-isa-sim/tree/v1.1.0#debugging-with-gdb
 
 [3].
-https://github.com/riscv/riscv-openocd
+https://github.com/riscv-collab/riscv-openocd