Enable Lint/BooleanSymbol in Rubocop for HAML

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

.haml-lint.yml

@@ -116,7 +116,6 @@ linters:
       - Layout/SpaceInsideHashLiteralBraces
       - Layout/SpaceInsideStringInterpolation
       - Layout/TrailingEmptyLines
-      - Lint/BooleanSymbol
       - Lint/LiteralInInterpolation
       - Lint/ParenthesesAsGroupedExpression
       - Lint/RedundantWithIndex

app/views/shared/issuable/_nav.html.haml

 - type = local_assigns.fetch(:type, :issues)
 - page_context_word = type.to_s.humanize(capitalize: false)
-- display_count = local_assigns.fetch(:display_count, :true)
+- display_count = local_assigns.fetch(:display_count, true)
 
 %ul.nav-links.issues-state-filters.mobile-separator.nav.nav-tabs
   %li{ class: active_when(params[:state] == 'opened') }>