Use ggsave instead of pdf - dev.off to save plots

[albhasan]

Remove the calls to pdf and dev.off to save plots in lesson 8. We use ggsave instead since it fits better with plots built using the ggplot2package.

This PR:

• Updates the text in lesson 8.
• Updates challenge 1 in lesson 8.

Closes #121

[github-actions]

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 08-writing-data.md | 63 +++++++++++++++++++++++++-----------------------------
 md5sum.txt         |  2 +-
 2 files changed, 30 insertions(+), 35 deletions(-)

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⏱️ Updated at 2024-01-11 14:01:20 +0000

[sstevens2]

This change looks good to me.

Optional addition might be showing learners how to save a plot to an object and then explicitly save a plot object. I've added something similar to my something I'm customizing of the genomics R lesson but I don't think it is necessary.