Update javascript.md

Updated the below line of code for npm setup based on customer feedback. Also validated this here - https://github.com/microsoft/ApplicationInsights-JS. 

appInsights.trackPageView(); // Manually call trackPageView to establish the current user/session/pageview

Author: BharathNimmala-MSFT

articles/azure-monitor/app/javascript.md

@@ -37,6 +37,7 @@ const appInsights = new ApplicationInsights({ config: {
   /* ...Other Configuration Options... */
 } });
 appInsights.loadAppInsights();
+appInsights.trackPageView(); // Manually call trackPageView to establish the current user/session/pageview
 ```
 
 ### Snippet based setup