web: fix missing title on WebPage_viewPreview

Use same label as the action, "Preview"
4 jobs for for_testrunner_2 in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-TestRunner2

07:34:32

passed ERP5.UnitTest-TestRunner2

09:34:56

passed ERP5.CodingStyleTest-TestRunner2

00:52:26

failed ERP5.UnitTest-TestRunner2

01:45:35