Doc: Remove \l from reference to example source file

Replace with \c since it is no longer possible
to put hard links to example source files

Change-Id: Iaf35f88a37b3c74455550316b83c26bc28769d3c
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>

examples/websockets/doc/echoserver.qdoc

@@ -59,7 +59,7 @@
     sends back the messages it receives, using the \l {Qt WebSockets}{WebSocket} API.
 
     If your web browser supports \l {Qt WebSockets}{WebSocket}, you can also use it
-    to open the \l {echoserver/echoclient.html}{echoclient.html} file, and operate
+    to open the \c {echoserver/echoclient.html}{echoclient.html} file, and operate
     like the following screenshot.
 
     \image echoclient-html-example.png Screenshot of the HTML version of Echo