Fixing the doc for symbolic version of rand_zipfian (apache#13978)

* Fixes apache#12779

python/mxnet/symbol/contrib.py

@@ -73,17 +73,14 @@ def rand_zipfian(true_classes, num_sampled, range_max):
 
     Examples
     --------
-    >>> true_cls = mx.nd.array([3])
-    >>> samples, exp_count_true, exp_count_sample = mx.nd.contrib.rand_zipfian(true_cls, 4, 5)
-    >>> samples
-    [1 3 3 3]
-    <NDArray 4 @cpu(0)>
-    >>> exp_count_true
-    [ 0.12453879]
-    <NDArray 1 @cpu(0)>
-    >>> exp_count_sample
-    [ 0.22629439  0.12453879  0.12453879  0.12453879]
-    <NDArray 4 @cpu(0)>
+    >>> true_cls = mx.sym.Variable('true_cls')
+    >>> samples, exp_count_true, exp_count_sample = mx.sym.contrib.rand_zipfian(true_cls, 4, 5)
+    >>> samples.eval(true_cls=mx.nd.array([3]))[0].asnumpy()
+    array([1, 3, 3, 3])
+    >>> exp_count_true.eval(true_cls=mx.nd.array([3]))[0].asnumpy()
+    array([0.12453879])
+    >>> exp_count_sample.eval(true_cls=mx.nd.array([3]))[0].asnumpy()
+    array([0.22629439, 0.12453879, 0.12453879, 0.12453879])
     """
     assert(isinstance(true_classes, Symbol)), "unexpected type %s" % type(true_classes)
     log_range = math.log(range_max + 1)