docs: make uv.kill's signum optional

[Bilal2453]

same as #666 for the same reasons.

See

luv/src/process.c

Lines 319 to 324 in 9e3c232

	static int luv_kill(lua_State* L) {
	int pid = luaL_checkinteger(L, 1);
	int signum = luv_parse_signal(L, 2);
	int ret = uv_kill(pid, signum);
	return luv_result(L, ret);
	}

[squeek502]

CI failure is unrelated: #669