doc: lisa.wa.WATraceCollector: Fix example error
Created by: mrkajetanp
The example given for lisa.wa.WATraceCollector currently passes an int to trace.analysis.idle.df_cluster_idle_state_residency(), which expects a list(int). Because of that, attempts to run the example fail.
Wrapping the cpu id in a list during the function call solves the issue without changing what the example is supposed to do.