doc: qemu-riscv: Fix opensbi build instructions

Latest opensbi uses generic platform for Qemu. Update the build
instructions.
Signed-off-by: NAtish Patra <atish.patra@wdc.com>
Reviewed-by: NBin Meng <bin.meng@windriver.com>
Reviewed-by: NRick Chen <rick@andestech.com>

doc/board/emulation/qemu-riscv.rst

@@ -73,7 +73,7 @@ supported by U-Boot. Clone the OpenSBI repository and run the following command.
 
     git clone https://github.com/riscv/opensbi.git
     cd opensbi
-    make PLATFORM=qemu/virt
+    make PLATFORM=generic
 
 See the OpenSBI documentation for full details:
 https://github.com/riscv/opensbi/blob/master/docs/platform/qemu_virt.md