tirith: fix case sensitive example

[bbigras]

LOW, MEDIUM, HIGH and CRITICAL are case sensitive

otherwise we get:

tirith: warning: failed to parse policy at /home/bbigras/.config/tirith/policy.yaml: severity_overrides.docker_untrusted_registry: unknown variant `critical`, expected one of `LOW`, `MEDIUM`, `HIGH`, `CRITICAL` at line 4 column 30
tirith: BLOCKED

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