README: make the demo code compilable

Before these changes, the example code in README.md would not compile (at least
on Fedora 40, gcc 14.1), mainly because of namespace issues.

After these changes, copying the demo code from README.md in a file named
"readme_demo.cpp" and executing the following commands works reliably:

EXAMPLE:
    # dnf install -y liburing-devel
    # g++ -Iinclude -std=c++20 -luring -o readme_demo readme_demo.cpp
    # ./readme_demo
    Hello world

README.md

@@ -15,13 +15,15 @@ Tested: `Ubuntu 5.9.0-050900rc6daily20200923-generic #202009222208 SMP Wed Sep 2
 ```cpp
 #include <liburing/io_service.hpp>
 
+using namespace std::literals;
+
 int main() {
     // You first need an io_service instance
-    io_service service;
+    uio::io_service service;
 
     // In order to `co_await`, you must be in a coroutine.
     // We use IIFE here for simplification
-    auto work = [&] () -> task<> {
+    auto work = [&] () -> uio::task<> {
         // Use Linux syscalls just as what you did before (except a little changes)
         const auto str = "Hello world\n"sv;
         co_await service.write(STDOUT_FILENO, str.data(), str.size(), 0);