Fix groupby example formatting

[ogonkov]

Summary

Proposed change:

One of the examples in #1276 is lack of code-fence.

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• Proposed change helps towards purpose of this project.
• Documentation is added / updated to describe proposed change.
• Tests are added / updated to cover proposed change.
• Changelog has an entry for proposed change (if user-facing fix or feature).

[codecov-commenter]

Codecov Report

Merging #1311 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1311   +/-   ##
=======================================
  Coverage   89.69%   89.69%           
=======================================
  Files          22       22           
  Lines        3046     3046           
=======================================
  Hits         2732     2732           
  Misses        314      314           

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