Fix name of the submodule entry.

Was `rack` in 1st edition but now it's `DbConnector`.

Author: zigarn

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

@@ -118,7 +118,7 @@ $ git commit -am 'added DbConnector module'
  create mode 160000 DbConnector
 ----
 
-Notice the `160000` mode for the rack entry.
+Notice the `160000` mode for the `DbConnector` entry.
 That is a special mode in Git that basically means you’re recording a commit as a directory entry rather than a subdirectory or a file.
 
 [[_cloning_submodules]]