README.md: adjust get_policy example output to match reality

[jacmet]

get_policy pretty prints information about the policy and not just the key
descriptor, so adjust the example in the README to match for clarity.

Signed-off-by: Peter Korsgaard peter@korsgaard.com

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[jacmet]

I signed it!

[googlebot]

CLAs look good, thanks!

[josephlr]

LGTM, thanks!