[Telemetry] Style fix, make docstrings consistent with style guide.

tools/telemetry/telemetry/timeline/async_slice.py

Index: tools/telemetry/telemetry/timeline/async_slice.py
diff --git a/tools/telemetry/telemetry/timeline/async_slice.py b/tools/telemetry/telemetry/timeline/async_slice.py
index 6216ed2f1e2b1fed788b413a09848d1d2cdbfcdf..2a5068fe618769578d30a5c4c4c15f5b3bbbb7f8 100644
--- a/tools/telemetry/telemetry/timeline/async_slice.py
+++ b/tools/telemetry/telemetry/timeline/async_slice.py
@@ -6,10 +6,10 @@ import telemetry.timeline.event as event
 
 
 class AsyncSlice(event.TimelineEvent):
-  ''' A AsyncSlice represents an interval of time during which an
+  """An AsyncSlice represents an interval of time during which an
   asynchronous operation is in progress. An AsyncSlice consumes no CPU time
   itself and so is only associated with Threads at its start and end point.
-  '''
+  """
   def __init__(self, category, name, timestamp, args=None,
                duration=0, start_thread=None, end_thread=None,
                thread_start=None, thread_duration=None):