Fix Zoekt disk usage graph

This MR fixes the Persistent Volume Disk Utilization dashboard. Currently, it uses ratios instead of percentages, which breaks the graph:

https://dashboards.gitlab.net/d/search-zoekt/search3a-zoekt-info?orgId=1&from=now-2d&to=now&viewPanel=7

This is how the graph is going to look after the fix: thanos

Test Dashboard

$ ./test-dashboard.sh search/zoekt.dashboard.jsonnet
Installed https://dashboards.gitlab.net/dashboard/snapshot/q7aZWa0OsMNNStlG41IHfYKEXOGBju27 - search: Zoekt Info

https://dashboards.gitlab.net/dashboard/snapshot/q7aZWa0OsMNNStlG41IHfYKEXOGBju27