Fix issue #557: Not precise statement for those two columns of git status -s

Author: YueLinHo

book/02-git-basics/sections/recording-changes.asc

@@ -184,7 +184,7 @@ M  lib/simplegit.rb
 ----
 
 New files that aren't tracked have a `??` next to them, new files that have been added to the staging area have an `A`, modified files have an `M` and so on.
-There are two columns to the output - the left hand column indicates that the file is staged and the right hand column indicates that it's modified.
+There are two columns to the output - the left-hand column indicates the status of the staging area and the right-hand column indicates the status of the working tree.
 So for example in that output, the `README` file is modified in the working directory but not yet staged, while the `lib/simplegit.rb` file is modified and staged.
 The `Rakefile` was modified, staged and then modified again, so there are changes to it that are both staged and unstaged.