Fix broken link to concept explanation for Mixer (#337)
Fix broken link to concept explanation for Mixer (#337)

… was trying to pop one too many parent directories.
istio/istio.io,ZackButcher/istio.github.io,jasminejaksic/istio.github.io,linsun/istio.github.io,istio/istio.io,ZackButcher/istio.github.io,geeknoid/istio.github.io,geeknoid/istio.github.io,linsun/istio.github.io,istio/istio.io,smawson/istio.github.io,mandarjog/mandarjog.github.io,linsun/istio.github.io,geeknoid/istio.github.io,smawson/istio.github.io,jasminejaksic/istio.github.io,mandarjog/mandarjog.github.io,istio/istio.io,mandarjog/mandarjog.github.io,jasminejaksic/istio.github.io,istio/istio.io,linsun/istio.github.io,smawson/istio.github.io,jasminejaksic/istio.github.io,mandarjog/mandarjog.github.io,geeknoid/istio.github.io,istio/istio.io,ZackButcher/istio.github.io,istio/istio.io,ZackButcher/istio.github.io,smawson/istio.github.io,geeknoid/istio.github.io,linsun/istio.github.io