doc: the aggregation example used group zero, which no longer parses

The command now numbers groups the way 'lag show' prints them, so the
walkthrough would have failed at its first step.

doc/link_aggregation.md

@@ -92,7 +92,7 @@ The following shows the network configuration
 On _both_ switches create a LAG with ports 1 and 2 inside and the default hash algorithm which takes
 source and destination ports into account, e.g. just use the default:
 ```
-> lag 0 1 2
+> lag 1 1 2
 ```