Update documentation for retrieving all templates (#764)

Add `generations` URI parameter to USAGE.md and example.

Author: tony-ho

USAGE.md

@@ -4167,7 +4167,7 @@ Transactional templates are templates created specifically for transactional ema
 
 
 ```csharp
-var response = await client.RequestAsync(method: SendGridClient.Method.GET, urlPath: "templates");
+var response = await client.RequestAsync(method: SendGridClient.Method.GET, urlPath: "templates?generations=legacy,dynamic");
 Console.WriteLine(response.StatusCode);
 Console.WriteLine(response.Body.ReadAsStringAsync().Result);
 Console.WriteLine(response.Headers.ToString());

examples/templates/templates.cs

@@ -26,7 +26,7 @@
 // Retrieve all transactional templates.
 // GET /templates
 
-var response = await client.RequestAsync(method: SendGridClient.Method.GET, urlPath: "templates");
+var response = await client.RequestAsync(method: SendGridClient.Method.GET, urlPath: "templates?generations=legacy,dynamic");
 Console.WriteLine(response.StatusCode);
 Console.WriteLine(response.Body.ReadAsStringAsync().Result);
 Console.WriteLine(response.Headers.ToString());