Update custom-proxy.md

updates to grammar

Author: cmastr

src/connections/sources/catalog/libraries/website/javascript/custom-proxy.md

@@ -6,7 +6,7 @@ strat: ajs
 
 Custom domains allow you to proxy Analytics.js and proxy all tracking event requests through your domain.
 
-The custom proxy setup for Analytics.js CDN or Tracking API do not apply to device-mode destinations since device-mode destinations require that the destination's native scripts get loaded onto the client, and their requests are sent directly to the destination. 
+You cannot use custom proxy setup for Analytics.js CDN or Tracking API with device-mode destinations because it requires the destination's native scripts are loaded onto the client, and the requests are sent directly to the destination. 
 
 ## Custom Proxy prerequisites