Update docs/configuring-playbook-matrix-corporal.md: remove the hr HTML elements used for styling

Now that the warning message is styled with the fancy decoration, it is no longer to wrap them with the hr HTML elements as they are redundant.

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-matrix-corporal.md

@@ -1,12 +1,8 @@
 # Setting up Matrix Corporal (optional, advanced)
 
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 > [!WARNING]
 > This is an advanced feature! It requires prior experience with Matrix and a specific need for using [Matrix Corporal](https://github.com/devture/matrix-corporal). If you're unsure whether you have such a need, you most likely don't.
 
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 The playbook can install and configure [matrix-corporal](https://github.com/devture/matrix-corporal) for you.
 
 In short, it's a sort of automation and firewalling service, which is helpful if you're instaling Matrix services in a controlled corporate environment.