Update default branch name for rstudio/cheatsheets (#6742)

* Update default branch name for rstudio/cheatsheets

rstudio/cheatsheets now has main as the default branch instead of master

* propagate to README.Rmd

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Co-authored-by: Teun van den Brand <tahvdbrand@gmail.com>

Author: abhro

README.Rmd

@@ -42,7 +42,7 @@ pak::pak("tidyverse/ggplot2")
 
 ## Cheatsheet
 
-<a href="https://github.com/rstudio/cheatsheets/blob/master/data-visualization.pdf"><img src="https://raw.githubusercontent.com/rstudio/cheatsheets/master/pngs/thumbnails/data-visualization-cheatsheet-thumbs.png" width="630" height="252" alt="ggplot2 cheatsheet" /></a>
+<a href="https://github.com/rstudio/cheatsheets/blob/main/data-visualization.pdf"><img src="https://raw.githubusercontent.com/rstudio/cheatsheets/main/pngs/thumbnails/data-visualization-cheatsheet-thumbs.png" width="630" height="252" alt="ggplot2 cheatsheet" /></a>
 
 ## Usage
 

README.md

@@ -35,7 +35,7 @@ pak::pak("tidyverse/ggplot2")
 
 ## Cheatsheet
 
-<a href="https://github.com/rstudio/cheatsheets/blob/master/data-visualization.pdf"><img src="https://raw.githubusercontent.com/rstudio/cheatsheets/master/pngs/thumbnails/data-visualization-cheatsheet-thumbs.png" width="630" height="252" alt="ggplot2 cheatsheet" /></a>
+<a href="https://github.com/rstudio/cheatsheets/blob/main/data-visualization.pdf"><img src="https://raw.githubusercontent.com/rstudio/cheatsheets/main/pngs/thumbnails/data-visualization-cheatsheet-thumbs.png" width="630" height="252" alt="ggplot2 cheatsheet" /></a>
 
 ## Usage