Fix clash with named anchor ID

Rename the anchor ID [[_default_branch]] to
[[_new_default_branch]], updating the reference
to it.

Fixes asciidoctor build warnings due to pre-existing
[[_default_branch]] anchor ID of the same name.

Author: rahrah

book/01-introduction/sections/first-time-setup.asc

@@ -84,7 +84,7 @@ You may find, if you don't setup your editor like this, you get into a really co
 An example on a Windows system may include a prematurely terminated Git operation during a Git initiated edit.
 ====
 
-[[_default_branch]]
+[[_new_default_branch]]
 ==== Your default branch name
 
 By default Git will create a branch called _master_ when you create a new repository with `git init`.

book/02-git-basics/sections/recording-changes.asc

@@ -40,7 +40,7 @@ For now, that branch is always `master`, which is the default; you won't worry a
 ====
 GitHub changed the default branch name from `master` to `main` in mid-2020, and other Git hosts followed suit.
 So you may find that the default branch name in some newly created repositories is `main` and not `master`.
-In addition, the default branch name can be changed (as you have seen in <<ch01-getting-started#_default_branch>>), so you may see a different name for the default branch.
+In addition, the default branch name can be changed (as you have seen in <<ch01-getting-started#_new_default_branch>>), so you may see a different name for the default branch.
 
 However, Git itself still uses `master` as the default, so we will use it throughout the book.
 ====