Add colon to SynBindingReturnInfo trivia.

[nojaf]

It is interesting to know the location of a : as it can be the first token of a new line according to the style guide.

    let longFunctionWithLotsOfParametersAndReturnType
        (aVeryLongParam: AVeryLongTypeThatYouNeedToUse)
        (aSecondVeryLongParam: AVeryLongTypeThatYouNeedToUse)
        (aThirdVeryLongParam: AVeryLongTypeThatYouNeedToUse)
        : ReturnType = ...