Update functions/imagemagick sample

[michaelawyu]

As requested in b/73815480.

[ace-n]

LGTM when:
a) you've verified that this works when deployed to GCF, and
b) the tests pass

[codecov]

Codecov Report

Merging #580 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #580   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines           3      3           
=====================================
  Hits            3      3

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