Fix admonitions invoked with #+begin_

[yamanq]

My previous pull request only works when the special boxes are invoked with tags. When using #+begin_note for example, the content is lost. These changes should work with both ways to create the special boxes.

Before:

After:

Source file

#+title: Test
#+SETUPFILE: org/theme-readtheorg-local.setup
* This is a test
I want to add some content before a note.
#+begin_note
To insert a note in the middle of headings, I use ~#+begin_note~ and ~#+end_note~
#+end_note
In addition to some content after the note
** Note created with tags :note:
This is a note created by adding ~:note:~ to a heading. I want it to be in the table of contents.

[fniessen]

Dear @yamanq, thanks for the fix!