2-contributing.asc: sed command fix/type

Author: apiemont

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

@@ -71,7 +71,7 @@ $ cd blink
 $ git checkout -b slow-blink <2>
 Switched to a new branch 'slow-blink'
 
-$ sed -i '' 's/1000/3000/' blink.ino <3>
+$ sed -i 's/1000/3000/' blink.ino <3>
 
 $ git diff --word-diff <4>
 diff --git a/blink.ino b/blink.ino