update doc to not use the root zone for everything (#3288)

Update all documentation in the tree to use example.org as an example
configuration (in so far possible). As to get out of the just use "."
and fallthrough and things would be fine.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/cache/README.md

@@ -93,13 +93,13 @@ Proxy to Google Public DNS and only cache responses for example.org (or below).
 }
 ~~~
 
-Enable caching for all zones, keep a positive cache size of 5000 and a negative cache size of 2500:
+Enable caching for `example.org`, keep a positive cache size of 5000 and a negative cache size of 2500:
 
 ~~~ corefile
- . {
-     cache {
-         success 5000
-         denial 2500
+example.org {
+    cache {
+        success 5000
+        denial 2500
     }
- }
- ~~~
+}
+~~~