perf_counter: update 'perf top' documentation

The documentation about the perf-top build was outdated after
perfstat has been implemented. This updates it.

[ Impact: update documentation ]
Signed-off-by: NRobert Richter <robert.richter@amd.com>
Cc: Paul Mackerras <paulus@samba.org>
Acked-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1241002046-8832-30-git-send-email-robert.richter@amd.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Documentation/perf_counter/builtin-top.c

@@ -3,7 +3,7 @@
 
    Build with:
 
-     cc -O6 -Wall -c -o kerneltop.o kerneltop.c -lrt
+     make -C Documentation/perf_counter/
 
    Sample output: