HttpCapabilitiesBase code example update (dotnet#5400)

The previous example created an error:
Cannot implicitly convert type 'System.Web.HttpBrowserCapabilitiesBase' to 'System.Web.HttpBrowserCapabilities'

The change to the example fixes that issue. 

I have tested the code and it now works as expected. I hope that my understanding of the issue is correct and that this is the correct change to make.

Author: OwainWilliams

samples/snippets/csharp/VS_Snippets_WebNet/HttpCapabilitiesBase.IsMobileDevice/CS/sample_cs.aspx

@@ -12,8 +12,8 @@
     void CheckBrowserCaps()
     {
         String labelText = "";
-        System.Web.HttpBrowserCapabilities myBrowserCaps = Request.Browser;
-        if (((System.Web.Configuration.HttpCapabilitiesBase)myBrowserCaps).IsMobileDevice)
+        System.Web.HttpBrowserCapabilitiesBase myBrowserCaps = Request.Browser;
+        if (myBrowserCaps.IsMobileDevice)
         {
             labelText = "Browser is a mobile device.";
         }
@@ -39,4 +39,4 @@
     </form>
 </body>
 </html>
-<!-- </Snippet1> -->
+<!-- </Snippet1> -->