Inconsistent use of 'uri' and 'url' in example/main.cpp

The `--help` option in line 25 lists `--url` as a possible argument
```c++
std::cout << "example --url [--protocol ] [--method ] [--arguments ] [--output ]\n";
```
but the argument parsing in line 28 expects `--uri`
```c++
else if (arg == "--uri")
```
and complains "Missing argument for --url" in line 31
```c++
else throw std::runtime_error("Missing argument for --url");
```