Fix parameter name in example script

[bwohlberg]

Correct on oversight from #249, which changes the parameter name of linop.Sum from sum_axis to axis.

[codecov]

Codecov Report

Merging #286 (40741bb) into main (157f2ba) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #286   +/-   ##
=======================================
  Coverage   94.15%   94.15%           
=======================================
  Files          49       49           
  Lines        3267     3267           
=======================================
  Hits         3076     3076           
  Misses        191      191           

Flag	Coverage Δ
unittests	94.15% <ø> (ø)

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