fix doc (#510)

nnvm/docs/dev/overview.md

@@ -103,7 +103,7 @@ void MyFunction() {
   static auto& finplace_option_map = Op::GetAttr<FInplaceOption>("FInplaceOption");
 
   // quick look up attribute of add, O(1) time, vector index lookup internally.
-  auto add_inplace = finplace_option_tbl[add];
+  auto add_inplace = finplace_option_map[add];
 }
 ```
 Besides making the code minimum, this attribute store enables decentralization of projects.

nnvm/include/nnvm/pass.h

@@ -47,7 +47,7 @@ inline Graph ApplyPass(Graph src, const std::string& pass) {
 
 
 /*!
- * \brief Registry entry for DataIterator factory functions.
+ * \brief Registry entry for pass functions.
  */
 struct PassFunctionReg
     : public dmlc::FunctionRegEntryBase<PassFunctionReg,