Update git diff explanation in history.md

episodes/05-history.md

@@ -62,7 +62,7 @@ index b36abfd..0848c8d 100644
 +An ill-considered change
 ```
 
-Note that `HEAD` is the default option for `git diff`, so omitting it will not change the command's output at all (give it a try). However, the real power of `git diff` lies in its ability to compare with previous commits. For example, by adding `~1` (where "~" is "tilde", pronounced [**til**\-d*uh*]), we can look at the commit before `HEAD`.
+Note that `HEAD` is the default option for `git diff` if there is no staged version, so omitting it will not change the command's output at all in this case (give it a try). If a staged file has been further modified, however, it will compare the modified version to the staged version (try adding guacamole.md to the staged area, making an additional modification, then running `git diff` and `git diff HEAD` again). In any case, the real power of `git diff` lies in its ability to compare with previous commits. For example, by adding `~1` (where "~" is "tilde", pronounced [**til**\-d*uh*]), we can look at the commit before `HEAD`.
 
 ```bash
 $ git diff HEAD~1 guacamole.md