Add space in "operator<<<...>" to avoid syntax error when targeting GPU

[fxzjshm]

CUDA/ROCm C++ has additional operator "<<< >>>" to invoke GPU kernel,
so compile error is given here:

In file included from /home/fxzjshm/workspace/project/deps/redis-plus-plus/src/sw/redis++/redis++.h:20:
In file included from /home/fxzjshm/workspace/project/deps/redis-plus-plus/src/sw/redis++/redis.h:29:
In file included from /home/fxzjshm/workspace/project/deps/redis-plus-plus/src/sw/redis++/subscriber.h:25:
In file included from /home/fxzjshm/workspace/project/deps/redis-plus-plus/src/sw/redis++/command.h:26:
/home/fxzjshm/workspace/project/deps/redis-plus-plus/src/sw/redis++/command_args.h:136:20: error: expected a type
  136 |     return operator<<<N + 1, Args...>(arg);
      |                    ^
/home/fxzjshm/workspace/project/deps/redis-plus-plus/src/sw/redis++/command_args.h:136:30: error: 'Args' does not refer to a value
  136 |     return operator<<<N + 1, Args...>(arg);
      |                              ^
/home/fxzjshm/workspace/project/deps/redis-plus-plus/src/sw/redis++/command_args.h:131:38: note: declared here
  131 | template <std::size_t N, typename ...Args>
      |                                      ^
2 errors generated when compiling for gfx1035.

added a space to avoid syntax error.