mediawiki: use contains to check if a string contains a substring

Change-Id: Ic98d295bb56c5466a388930b6cea403d6307b850
Reviewed-on: https://gerrit.libreoffice.org/11298
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>