update workflow diagram

- rename gitlab to github
- grey (optional) RStudio
- redraw directional lines/arrows

Author: libjohn

slides/images/git_workflow.png

@@ -55,9 +55,9 @@
 
 Use Git to manage version control
 
-Add • Commit • Branch • Merge • Revert
+Clone • Pull • Add • Commit • Push 
 
-(Pull • Push • Clone • Fork • Collaborate)
+(Branch • Merge • Revert • Fork • Collaborate)
 
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 class: middle, center