Use true instead of symbol in HAML

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

ee/app/views/shared/issuable/_epic_nav.html.haml

 - type = local_assigns.fetch(:type, :epics)
 - 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.mobile-separator.epics-state-filters
   %li{ class: active_when(params[:state] == 'opened') }>