Fix type annotation in sample code for Decode.dict

[rtfeldman]

The docs for Decode.dict had an inaccurate type annotation in their code sample (showing Int instead of Float), which this fixes.

Before:

-- { mercury: 0.33, venus: 4.87, earth: 5.97, ... }
planetMasses : Decoder (Dict String Int)
planetMasses =
    dict float

After:

-- { mercury: 0.33, venus: 4.87, earth: 5.97, ... }
planetMasses : Decoder (Dict String Float)
planetMasses =
    dict float

[evancz]

Nice catch, thanks!

[evancz]

Nice catch, thanks!