Doc: Add HTTP Client example also to Web Technologies category

HTTP belongs to Networking, but also fits well with the scope
of Web Technologies - qtnetworkserver is there after all, too.

So just add the example to both.

Pick-to: 6.6 6.6.0
Change-Id: I588e4e0eefc92ba396fbb21d6f55b33f68332a74
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

examples/network/doc/src/http.qdoc

@@ -4,6 +4,7 @@
 /*!
     \example http
     \examplecategory {Networking}
+    \examplecategory {Web Technologies}
     \meta tags {http,network,https,proxy}
     \title HTTP Client
     \ingroup examples-network