perf intel-pt: Fix syntax in documentation of config option

As specified in tools/perf/Documentation/perf-config.txt, perf
configuration items must be in 'key = value' format, otherwise the
following error message occurs:

  $ perf record -e intel_pt//u -- ls
  bad config file line 2 in ~/.perfconfig
  $ cat .perfconfig
  [intel-pt]
      mispred-all

Changing to assigning a value to the key 'mispred-all' fixes the issue:

  $ perf record -e intel_pt//u -- ls
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Capured and wrote 0.031 MB perf.data]
  $ cat .perfconfig
  [intel-pt]
      mispred-all = true
Signed-off-by: NJack Henschel <jackdev@mailbox.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170831080535.2157-1-jackdev@mailbox.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/intel-pt.txt

@@ -873,7 +873,7 @@ amended to take the number of elements as a parameter.
 
 	$ cat ~/.perfconfig
 	[intel-pt]
-		mispred-all
+		mispred-all = on
 
 	$ perf record -e intel_pt//u ./sort 3000
 	Bubble sorting array of 3000 elements