Correct metrics in sizing-feeding doc

[bratseth]

https://docs.vespa.ai/en/performance/sizing-feeding.html lists metrics that doesn't appear to exist any more.

At least

vds.filestor.alldisks.averagequeuewait.sum
vds.filestor.alldisks.queuesize
vds.filestor.alldisks.allthreads.update.sum.latency

[kkraune]

I'll take a stab at this once the 8 Docker image is out - there are some changes in https://docs.vespa.ai/en/vespa8-release-notes.html like

vds.filestor.alldisks.* vds.filestor.* alldisks has been removed from the metric name.

so the ticket is accurate