Standardize on " -- " to represent "em" dashes in "Undoing Things".

Author: robertpjday

book/02-git-basics/sections/undoing.asc

@@ -29,7 +29,7 @@ $ git add forgotten_file
 $ git commit --amend
 ----
 
-You end up with a single commit – the second commit replaces the results of the first.
+You end up with a single commit -- the second commit replaces the results of the first.
 
 [NOTE]
 ====
@@ -97,7 +97,7 @@ We'll go into much more detail about what `reset` does and how to master it to d
 ==== Unmodifying a Modified File
 
 What if you realize that you don't want to keep your changes to the `CONTRIBUTING.md` file?
-How can you easily unmodify it – revert it back to what it looked like when you last committed (or initially cloned, or however you got it into your working directory)?
+How can you easily unmodify it -- revert it back to what it looked like when you last committed (or initially cloned, or however you got it into your working directory)?
 Luckily, `git status` tells you how to do that, too.
 In the last example output, the unstaged area looks like this:
 
@@ -130,7 +130,7 @@ You can see that the changes have been reverted.
 [IMPORTANT]
 =====
 It's important to understand that `git checkout -- <file>` is a dangerous command.
-Any changes you made to that file are gone – Git just copied another file over it.
+Any changes you made to that file are gone -- Git just copied another file over it.
 Don't ever use this command unless you absolutely know that you don't want the file.
 =====