use full text in fill_in at Issue system "update issue with form update" test

git-svn-id: http://svn.redmine.org/redmine/trunk@18661 e93f8b46-1217-0410-a6f0-8f06a7374b81

test/system/issues_test.rb

@@ -210,7 +210,7 @@ class IssuesTest < ApplicationSystemTestCase
     select 'Feature request', :from => 'Tracker'
     assert page.has_content?('Form update CF')
 
-    fill_in 'Form update', :with => 'CF value'
+    fill_in 'Form update CF', :with => 'CF value'
     assert_no_difference 'Issue.count' do
       page.first(:button, 'Submit').click
     end