add doc for erf into index, test=document_preview (#1759)

add doc for erf into index

doc/fluid/api/layers.rst

@@ -96,6 +96,7 @@ fluid.layers
     layers/elu.rst
     layers/embedding.rst
     layers/equal.rst
+    layers/erf.rst
     layers/exp.rst
     layers/expand.rst
     layers/expand_as.rst

doc/fluid/api_cn/layers_cn.rst

@@ -101,6 +101,7 @@ fluid.layers
     layers_cn/elu_cn.rst
     layers_cn/embedding_cn.rst
     layers_cn/equal_cn.rst
+    layers_cn/erf_cn.rst
     layers_cn/exp_cn.rst
     layers_cn/expand_cn.rst
     layers_cn/expand_as_cn.rst