Fix truncated NatSpec for avg function

[ericglau]

The NatSpec for the avg function is missing the @notice part because one of the lines is // instead of ///. This causes the function's documentation to only start from In English, ...

For example, the AST currently has the following:

        "documentation": {
          "id": 26595,
          "nodeType": "StructuredDocumentation",
          "src": "779:614:40",
          "text": "In English, ..."
        },

[PaulRBerg]

Thanks, Eric.