Update 07-vector-shapefile-attributes-in-r.Rmd

[rfbrown]

Please briefly summarise the changes made in the pull request, and the reason(s) for making these changes.

In the line width example, the size aesthetic doesn't work (i.e., the lines are all the same size). We updated to linewidth as a proposed solution. However, we get the following warning (which also appears when using size), so this may not be the best fix: "Warning message: Using linewidth for a discrete variable is not advised."

[github-actions]

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[rfbrown]

Note that this issue shows up again in the challenge, so the fix should be consistently applied throughout the lesson.

[Karel-Kroeze]

the warning is correct, it doesn't make a lot of sense to use a discrete value (that could be ordered in terms of 'size' of path, but isn't) and map it onto what is by convention if not definition an aesthetic usually used for continuous or at least ordered data.

That said, for illustrative purposes it's fine, and it might even be a good segue into letting learners think a bit about how best to visualize different types of attributes, and letting them know that ggplot2 has a lot of (usually very reasonable) opinions and defaults.