Update 2-contributing.asc

reformatting sed linux command, as per Ben suggestion

Author: apiemont

book/06-github/sections/2-contributing.asc

@@ -72,7 +72,8 @@ $ git checkout -b slow-blink <2>
 Switched to a new branch 'slow-blink'
 
 $ sed -i '' 's/1000/3000/' blink.ino <3> (MacOSX)
-$ sed -i 's/1000/3000/' blink.ino <3> (Linux)
+# If you're on a Linux system, do this instead:
+# $ sed -i 's/1000/3000/' blink.ino
 
 $ git diff --word-diff <4>
 diff --git a/blink.ino b/blink.ino