[doc] Fix qemu-riscv64 command line flag (#34866)

### Details:
- riscv-collab toolchain uses updated flag for enabling vector extension
(RVV) usage on qemu-riscv64 emulator

### Tickets:
 - N/A

### AI Assistance:
 - *AI assistance used: no*

Co-authored-by: Nesterov Alexander <alexander.nesterov@intel.com>

Author: aobolensk

docs/dev/build_riscv64.md

@@ -120,12 +120,12 @@ In order to test applications without hardware one can use emulation software. T
 
 For example, to emulate RVV 0.7.1:
 ```sh
-<riscv_toolchain_root>/bin/qemu-riscv64 -cpu rv64,x-v=true,vext_spec=v0.7.1 <executable_file_path>
+<riscv_toolchain_root>/bin/qemu-riscv64 -cpu rv64,v=true,vext_spec=v0.7.1 <executable_file_path>
 ```
 
 Or to emulate RVV 1.0:
 ```sh
-<riscv_toolchain_root>/bin/qemu-riscv64 -cpu rv64,x-v=true,vext_spec=v1.0 <executable_file_path>
+<riscv_toolchain_root>/bin/qemu-riscv64 -cpu rv64,v=true,vext_spec=v1.0 <executable_file_path>
 ```
 
 ## See also