Correct the wrong example about JSON source generator in the readme

[KodamaSakuno]

What kind of change does this PR introduce?
Correct the wrong example about JSON source generator in the readme.

Other information:
The following code shows the difference.

public class UnitTest
{
    private readonly HttpClient _client = new(new MyHttpMessageHandler());

    [Fact]
    public async Task Test1()
    {
        Response? response = null;
        
        var options = MyJsonSerializerContext.Default.Options;
        
        var exception = await Record.ExceptionAsync(async () =>
        {
            response = await _client.GetFromJsonAsync<Response>("http://abc", options);
        });
        
        Assert.Null(exception);
        Assert.Equal(1234, response?.Code);
    }
    
    [Fact]
    public async Task Test2()
    {
        Response? response = null;
        
        var options = new JsonSerializerOptions()
        {
            TypeInfoResolver = MyJsonSerializerContext.Default,
        };
        
        var exception = await Record.ExceptionAsync(async () =>
        {
            response = await _client.GetFromJsonAsync<Response>("http://abc", options);
        });
        
        Assert.IsType<JsonException>(exception);
        Assert.StartsWith("The JSON value could not be converted to System.Int32. Path: $.Code", exception.Message);
    }
}

public class MyHttpMessageHandler : HttpMessageHandler
{
    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        return Task.FromResult(new HttpResponseMessage()
        {
            Content = new StringContent("{\"Code\": \"1234\"}"),
        });
    }
}
    
public class Response
{
    public int Code { get; set; }
}

[JsonSourceGenerationOptions(NumberHandling = JsonNumberHandling.AllowReadingFromString)]
[JsonSerializable(typeof(Response))]
public partial class MyJsonSerializerContext : JsonSerializerContext;

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.03%. Comparing base (6ebeda5) to head (28acb43).
Report is 190 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1983      +/-   ##
==========================================
- Coverage   87.73%   83.03%   -4.71%     
==========================================
  Files          33       37       +4     
  Lines        2348     2640     +292     
  Branches      294      380      +86     
==========================================
+ Hits         2060     2192     +132     
- Misses        208      362     +154     
- Partials       80       86       +6     

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[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.