fix pdf link

Signed-off-by: Mengxin Liu <liumengxinfly@gmail.com>

Author: oilbeater

overrides/contact.md

@@ -1,3 +1,3 @@
-[:material-download: PDF](/docs/v1.15.x/en/pdf/document.pdf){: target="_blank" .md-button }
+[:material-download: PDF](/docs/v1.15.x/pdf/document.pdf){: target="_blank" .md-button }
 [:material-slack: Slack](https://communityinviter.com/apps/kube-ovn/kube-ovn/){: target="_blank" .md-button }
 [:material-email: Support](https://www.alauda.io/products/kube-ovn){: target="_blank" .md-button }