Update first-web-api.md

Author: Cam Soper

aspnetcore/tutorials/first-web-api.md

@@ -125,6 +125,7 @@ The previously highlighted code:
 * Sets the `SwaggerEndpoint()` option to the location of the OpenAPI documentation for this project.  
 * Ensures the Swagger UI is only available in the app development environment to limit information disclosure and security vulnerability.
 
+```csharp
 var app = builder.Build();
 
 // Configure the HTTP request pipeline.
@@ -562,4 +563,4 @@ For more information, see the following resources:
 
 [!INCLUDE[](~/tutorials/first-web-api/includes/first-web-api7.md)]
 
-[!INCLUDE[](~/tutorials/first-web-api/includes/first-web-api8.md)]
+[!INCLUDE[](~/tutorials/first-web-api/includes/first-web-api8.md)]