perf script python: Fix wrong code snippets in documentation

This commit fixes wrong code snippets for trace_begin() and trace_end()
function example definition.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tom Zanussi <tzanussi@gmail.com>
Fixes: cff68e5822 ("perf/scripts: Add perf-trace-python Documentation")
Link: http://lkml.kernel.org/r/20170530111827.21732-5-sj38.park@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-script-python.txt

@@ -532,7 +532,7 @@ can implement a set of optional functions:
 gives scripts a chance to do setup tasks:
 
 ----
-def trace_begin:
+def trace_begin():
     pass
 ----
 
@@ -541,7 +541,7 @@ def trace_begin:
  as display results:
 
 ----
-def trace_end:
+def trace_end():
     pass
 ----