Remove reliance on implicit df behavior, #819

[brshallo]

Closes #819

The example I changed relied on implicit behavior of dataframes (which is often better to avoid and doesn't work on tibbles) so I made a slight change to the example that results in the same output but doesn't rely on the implicit dataframe behavior of converting a one column dataframe into a vector.

[github-actions]

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🔍 Inspect the changes: https://github.com/swcarpentry/r-novice-gapminder/compare/md-outputs..md-outputs-PR-866

The following changes were observed in the rendered markdown documents:

 13-dplyr.md | 6 +++---
 md5sum.txt  | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

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⏱️ Updated at 2023-11-15 23:52:51 +0000

[matthieu-bruneaux]

Thank you for your contribution @brshallo! This is very helpful.