Update rewrite-http-headers-url.md

AppGW Rewrite Rules do not support 'X-Original-Host' under the "Common Header" > "Set", it offers other X-headers, but not Original Host.
Additionally, if using "Custom Header", it allows you to create the Rewrite Rule, but does not take effect since the AppGW 'creates' this header (non-standard HTTP Header)

Author: BrandynR-MSFT

articles/application-gateway/rewrite-http-headers-url.md

@@ -256,7 +256,7 @@ For a URL redirect, Application Gateway sends a redirect response to the client
 - Rewrites aren't supported when the application gateway is configured to redirect the requests or to show a custom error page.
 - Request header names can contain alphanumeric characters and hyphens. Headers names containing other characters will be discarded when a request is sent to the backend target.
 - Response header names can contain any alphanumeric characters and specific symbols as defined in [RFC 7230](https://tools.ietf.org/html/rfc7230#page-27).
-- Connection and upgrade headers cannot be rewritten
+- X-Original-Host, Connection, and upgrade headers cannot be rewritten
 - Rewrites aren't supported for 4xx and 5xx responses generated directly from Application Gateway
 
 ## Next steps