Apply Layout/SpaceInLambdaLiteral rubocop rule

I had mixed feelings about this rule, since I like spaces where
possible.

However, I changed my mind when I realized writing `->(thing) { }` was
similar to defining a method, and we don't have a space before the
parenthesis when defining a method.

app/models/poll/partial_result.rb

@@ -9,7 +9,7 @@ class Poll::PartialResult < ApplicationRecord
   validates :question, presence: true
   validates :author, presence: true
   validates :answer, presence: true
-  validates :answer, inclusion: { in: -> (a) { a.question.possible_answers }},
+  validates :answer, inclusion: { in: ->(a) { a.question.possible_answers }},
                      unless: ->(a) { a.question.blank? }
   validates :origin, inclusion: { in: VALID_ORIGINS }