Merge pull request #7028 from Icinga/bugfix/doc-heading-level

Fix heading level in development chapter

diff --cc doc/21-development.md
index e4c277a2294c249d1e7ff85ce677e34042730edd,1c872bb759ae7ce7436b65b94c3c7eccee88c392..c5d1e4af8c966f9f12ea27d472579c34de65d947
--- 1/doc/21-development.md
--- 2/doc/21-development.md
+++ b/doc/21-development.md
@@@ -634,10 -634,10 +634,10 @@@ vim /usr/local/icinga2/etc/icinga2/conf
  gdb --args /usr/local/icinga2/lib/icinga2/sbin/icinga2 daemon
  ```
  
- ##### Debian 9 <a id="development-linux-dev-env-debian"></a>
+ #### Debian 9 <a id="development-linux-dev-env-debian"></a>
  
  ```
 -apt-get -y install gdb vim git cmake make ccache build-essential libssl-dev libboost-all-dev bison flex default-libmysqlclient-dev libpq-dev libyajl-dev libedit-dev monitoring-plugins
 +apt-get -y install gdb vim git cmake make ccache build-essential libssl-dev libboost-all-dev bison flex default-libmysqlclient-dev libpq-dev libedit-dev monitoring-plugins
  
  ln -s /usr/bin/ccache /usr/local/bin/gcc
  ln -s /usr/bin/ccache /usr/local/bin/g++