fix links to metrics notebooks

[RafalSkolasinski]

What this PR does / why we need it:

Fixes two broken links.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

None

[seldondev]

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[seldondev]

Wed Oct 14 16:16:20 UTC 2020
The logs for [lint] [2] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2548/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2548 --build=2

[seldondev]

Wed Oct 14 16:16:21 UTC 2020
The logs for [pr-build] [1] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2548/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2548 --build=1