Add missing attribute

Add `[NotNull]` to `ZohoAuthenticationHandler.ExchangeCodeAsync()` to fix `Publicly_Visible_Parameters_Have_Null_Annotations`.

src/AspNet.Security.OAuth.Zoho/ZohoAuthenticationHandler.cs

@@ -53,7 +53,7 @@ protected override async Task<AuthenticationTicket> CreateTicketAsync(
         return new AuthenticationTicket(context.Principal!, context.Properties, Scheme.Name);
     }
 
-    protected override async Task<OAuthTokenResponse> ExchangeCodeAsync(OAuthCodeExchangeContext context)
+    protected override async Task<OAuthTokenResponse> ExchangeCodeAsync([NotNull] OAuthCodeExchangeContext context)
     {
         var nameValueCollection = new Dictionary<string, string?>
         {