Increase the time we wait for the link to appear

Error message: timed out after 15 seconds, waiting for
{:text=>"Download the file", :tag_name=>"a"} to become present
(Watir::Wait::TimeoutError)

Failed Jenkins job:
https://wmf.ci.cloudbees.com/view/s-e.w-l/job/browsertests-en.wikipedia.
beta.wmflabs.org-linux-chrome/103/testReport/(root)/PDF/Click_on_Downloa
d_as_PDF_link/

Change-Id: Ibbdce47cd1e8fccbc95eefaa9851250093e5159b