Submodules: update 'git submodule update --remote' behaviour

Since f0a96e8d4c (submodule: fall back to remote's HEAD for missing
remote.<name>.branch, 2020-06-24), which went into Git 2.28, 'git
submodule upadte --remote' does not default to using the 'master' branch
of the remote repository, but rather the remote repository's HEAD
branch.

Tweak the wording to reflect this behaviour change.

Author: phil-blain

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

@@ -262,7 +262,7 @@ From https://github.com/chaconinc/DbConnector
 Submodule path 'DbConnector': checked out 'd0354fc054692d3906c85c3af05ddce39a1c0644'
 ----
 
-This command will by default assume that you want to update the checkout to the `master` branch of the submodule repository.
+This command will by default assume that you want to update the checkout to the default branch of the remote submodule repository (the one pointed to by `HEAD` on the remote).
 You can, however, set this to something different if you want.
 For example, if you want to have the DbConnector submodule track that repository's ``stable'' branch, you can set it in either your `.gitmodules` file (so everyone else also tracks it), or just in your local `.git/config` file.
 Let's set it in the `.gitmodules` file: