Fix a grammatical mistake in submodules.asc

Author: brettcannon

book/07-git-tools/sections/submodules.asc

@@ -374,7 +374,7 @@ So now let's go through an example of making changes to the submodule at the sam
 
 So far, when we've run the `git submodule update` command to fetch changes from the submodule repositories, Git would get the changes and update the files in the subdirectory but will leave the sub-repository in what's called a ``detached HEAD'' state.
 This means that there is no local working branch (like ``master'', for example) tracking changes.
-So any changes you make aren't being tracked well.
+So any changes you make aren't being tracked as well.
 
 In order to set up your submodule to be easier to go in and hack on, you need do two things.
 You need to go into each submodule and check out a branch to work on.