fix typo

hapi/metrics.py

@@ -54,7 +54,7 @@ class Metric(object):
 
         Inputs of :code:`update` is the outputs of :code:`add_metric_op`, if
         :code:`add_metric_op` is not defined, the inputs of :code:`update` will
-        be a flatten argument of **output** of mode and **label** from data:
+        be flatten arguments of **output** of mode and **label** from data:
         :code:`update(output1, output2, ..., label1, label2,...)`
 
         see :code:`add_metric_op`