Fix string format substitution key missing error

jayvdb · jayvdb · commit b8f127bc0f31 · 2015-12-21T13:09:31.000+11:00
Replace `{..}` literal as `{{..}}` to avoid key substitution error:

    KeyError: "'h', 'v'"
diff --git a/examples/api/filled_step.py b/examples/api/filled_step.py
@@ -41,7 +41,7 @@ def filled_hist(ax, edges, values, bottoms=None, orientation='v',
     """
     print(orientation)
     if orientation not in set('hv'):
-        raise ValueError("orientation must be in {'h', 'v'} "
+        raise ValueError("orientation must be in {{'h', 'v'}} "
                          "not {o}".format(o=orientation))
 
     kwargs.setdefault('step', 'post')
