Correct icon used by #preview_link helper

The icon class name will be renamed in a future WP/PR:
https://community.openproject.org/work_packages/19820

Signed-off-by: Alex Coles <alex@alexbcoles.com>

app/helpers/previews_helper.rb

@@ -34,8 +34,8 @@ module PreviewsHelper
       id: link_id,
       'has-preview' => '',
       # NOTE:   legacy JS relies on preview class
-      # FIXME:  replace placeholder icon with preview icon
-      class: 'button preview -with-icon icon-ticket-checked'
+      # FIXME:  fix preview icon naming
+      class: 'button preview -with-icon icon-issue-watched'
     }.merge(options)
 
     link_to path, options do

spec/helpers/previews_helper_spec.rb

@@ -42,7 +42,7 @@ describe PreviewsHelper, type: :helper do
     it 'outputs a styled link' do
       expect(output).to be_html_eql(%{
         <a href="/"
-          class="button preview -with-icon icon-ticket-checked"
+          class="button preview -with-icon icon-issue-watched"
           accesskey="a"
           has-preview=""
           id="news_preview">Preview</a>