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

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

Update element_rect(size = 1) to element_rect(line_width = 1) to address warning: The size argument of element_rect() is deprecated as of ggplot2 3.4.0. ℹ Please use the linewidth argument instead.

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We note that the warning also shows up in the lesson and can be used as an example (which we did on the fly)- but if used as an example, the lesson should contain descriptive text about the warning.

Note that there are a few other places in the lesson where this issue shows up, so the fix should be consistent. We suggest changing all mentions of element_rect(size = ) to element_rect(linewidth = ).