Update free-programming-cheatsheets.md

Author: chrdek

Diff for: more/free-programming-cheatsheets.md

@@ -166,6 +166,7 @@
 
 * [All RStudio cheatsheets resources](https://www.rstudio.com/resources/cheatsheets) - RStudio.com (HTML site with PDF links)
 
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 ### Raspberry Pi
 
 * [Top 50 General Commands for Raspberry Pi cheatsheet](https://projects-raspberry.com/wp-content/uploads/2018/05/Top-50-Raspberry-pi-commands-List-cheat-sheet.pdf) - Projects Raspberry (PDF)
@@ -174,6 +175,7 @@
 * [Raspberry Pi Different GPIO Configuration Combinations cheatsheet](https://static.raspberrypi.org/files/education/posters/GPIO_Zero_Ceatsheet.pdf) - GPIO Zero Cheatsheet (PDF)
 * [Basic GPIO layout configuration cheatsheet](https://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/robot/cheat_sheet/) - University of Cambridge Computer Laboratory Raspberry Pi Projects Cheatsheet (PDF)
 
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 ### Ruby
 
 * [Ruby Cheat Sheet](https://www.codeconquest.com/wp-content/uploads/Ruby-Cheat-Sheet-by-CodeConquestDOTcom.pdf) - CodeConquest.com (PDF)