Fix invalid style for listing caution block

Author: HonkingGoose

book/03-git-branching/sections/branch-management.asc

@@ -77,10 +77,10 @@ $ git branch --no-merged master
 ==== Changing a branch name
 
 [CAUTION]
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+====
 Do not rename branches that are still in use by other collaborators.
 Do not rename a branch like master/main/mainline without having read the section "Changing the master branch name".
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+====
 
 Suppose you have a branch that is called _bad-branch-name_ and you want to change it to _corrected-branch-name_, while keeping all history.
 You also want to change the branch name on the remote (GitHub, GitLab, other server).