Fix rrule of sum3 example

[niklasschmitz]

I enjoyed reading this clear doc page! In the end fixed an rrule typo I think, timings and results on the example are left unchanged 👍

[codecov-io]

Codecov Report

Merging #307 (2dbd80c) into master (f81ccda) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #307   +/-   ##
=======================================
  Coverage   89.40%   89.40%           
=======================================
  Files          13       13           
  Lines         472      472           
=======================================
  Hits          422      422           
  Misses         50       50           

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[oxinabox]

Oh good catch!