Fix wrong usage of doxygen groups (#1417)

include/json/value.h

@@ -437,23 +437,23 @@ class JSON_API Value {
   /// \post type() is arrayValue
   void resize(ArrayIndex newSize);
 
-  //@{
+  ///@{
   /// Access an array element (zero based index). If the array contains less
   /// than index element, then null value are inserted in the array so that
   /// its size is index+1.
   /// (You may need to say 'value[0u]' to get your compiler to distinguish
   /// this from the operator[] which takes a string.)
   Value& operator[](ArrayIndex index);
   Value& operator[](int index);
-  //@}
+  ///@}
 
-  //@{
+  ///@{
   /// Access an array element (zero based index).
   /// (You may need to say 'value[0u]' to get your compiler to distinguish
   /// this from the operator[] which takes a string.)
   const Value& operator[](ArrayIndex index) const;
   const Value& operator[](int index) const;
-  //@}
+  ///@}
 
   /// If the array contains at least index+1 elements, returns the element
   /// value, otherwise returns defaultValue.