jsonrpc.md: Fix code block fence

Change-Id: Iaa569482ada24976506d44f7dc1fe95004653c3d
Signed-off-by: Alexey Marchuk <alexeymar@mellanox.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/4331
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>

doc/jsonrpc.md

@@ -7327,7 +7327,7 @@ Example response:
   "id": 1,
   "result": "true"
 }
-~~
+~~~
 
 ## blobfs_set_cache_size {#rpc_blobfs_set_cache_size}