Intro to R Fixes

[cansavvy]

This PR closes #110.

For this module, this leaves only the more diagrams issue and adding more simple exercises issue #125 to be addressed in this module.

This PR includes a lot of little changes. If this feels like too much to review at once, let me know and I can try to chunk it more. But to make it more clear the changes I've made; here's the list:

Typo fixes

• Got rid of extra space in df <- data.frame(nums , words, color_vector)
• *FE*males who are older than 100 months
• Bullet spacing for resources is fixed
• Indent of "vector" bullet point

Organizational changes

• Re-order read in metadata.tsv AFTER using file.exist
• Move question "Why do you suppose we don't have a comma to subset this object?" to after the code chunk - @cbethell 's suggestion

Explanation adds

• Explain a TSV file, CSV file, RDS file
• Explain %in%
• Explain dplyr::join
• Explain negative indices

PR Checklist

• Run a linter - (NA Most of the adds are words and not code so a linter would not help much here)
• Set the seed (if applicable)
• Comments and/or documentation up to date
• Double check your paths (NA - no extra files are added)
• Check headline numbering (NA - sections are not numbered)
• Spell check any Rmd file or md file
• Restart R and run all notebooks fresh and save

[cbethell]

LGTM! The subset question is much more clear, and so are the explanations you added.

[jaclyn-taroni]

LGTM. Let's make sure to get that plot displayed before merging.