Update free-programming-cheatsheets.md

Author: chrdek

more/free-programming-cheatsheets.md

@@ -168,11 +168,11 @@
 
 ### 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)
-* [Raspberry Pi Basics cheatsheet](https://www.woolseyworkshop.com/wp-content/uploads/WoolseyWorkshop_Cheatsheet_RaspberryPiBasics_v1.4.pdf)
-* [Other Raspberry Pi Commands cheatsheet](https://www.raspberrypistarterkits.com/wp-content/uploads/2018/01/raspberry-pi-commands-cheat-sheet.pdf)
-* [Raspberry Pi Different GPIO Configuration Combinations cheatsheet](https://static.raspberrypi.org/files/education/posters/GPIO_Zero_Ceatsheet.pdf)
-* [Basic GPIO layout configuration cheatsheet](https://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/robot/cheat_sheet/)
+* [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)(PDF)
+* [Raspberry Pi Basics cheatsheet](https://www.woolseyworkshop.com/wp-content/uploads/WoolseyWorkshop_Cheatsheet_RaspberryPiBasics_v1.4.pdf)(PDF)
+* [Other Raspberry Pi Commands cheatsheet](https://www.raspberrypistarterkits.com/wp-content/uploads/2018/01/raspberry-pi-commands-cheat-sheet.pdf)(PDF)
+* [Raspberry Pi Different GPIO Configuration Combinations cheatsheet](https://static.raspberrypi.org/files/education/posters/GPIO_Zero_Ceatsheet.pdf)(PDF)
+* [Basic GPIO layout configuration cheatsheet](https://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/robot/cheat_sheet/)(PDF)
 
 ### Ruby