Update documentation in metrics/__init__.py

BUG=391943
TBR=nduca@chromium.org

Review URL: https://codereview.chromium.org/382193003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282670 0039d316-1c4b-4281-b951-d872f2087c98

tools/perf/metrics/__init__.py

@@ -36,6 +36,6 @@ class Metric(object):
 
     Metrics may implement AddResults to provide a common way to add results
     to the PageMeasurementResults in PageMeasurement.AddMeasurement --
-    results should be added with results.Add(trace_name, unit, value).
+    results should be added with results.AddValue(...).
     """
     raise NotImplementedError()