Utterance "winter 2020" not parsed correctly.

Utterance `winter 2020` is broken down into two entities - `winter` and `2020`. For all other seasons, the correct date range is returned (e.g. `summer 2020`, `fall 2021`, etc.).

Having a hard time finding the right rules to edit on this one. Assuming it has something to do with these lines:
```
ruleSeason :: Rule
ruleSeason = Rule
{ name = "last|this|next "
, pattern =
[ regex "(this|current|next|last|past|previous) seasons?"
]
, prod = \case
(Token RegexMatch (GroupMatch (match:_)):_) -> do
n <- case Text.toLower match of
"this" -> Just 0
"current" -> Just 0
"last" -> Just (-1)
"past" -> Just (-1)
"previous" -> Just (-1)
"next" -> Just 1
_ -> Nothing
tt $ predNth n False season
_ -> Nothing
}

ruleSeasons :: [Rule]
ruleSeasons = mkRuleSeasons
[ ( "summer", "summer" , monthDay 6 21, monthDay 9 23 )
, ( "fall" , "fall|autumn", monthDay 9 23, monthDay 12 21 )
, ( "winter", "winter" , monthDay 12 21, monthDay 3 20 )
, ( "spring", "spring" , monthDay 3 20, monthDay 6 21 )
]
```
`in lines 956-982 of Duckling/Time/En/Rules.hs`