Fix heading level in development chapter

This fixes the wrong heading level for the Debian development
environment.

(cherry picked from commit da3775b6dbc0541558b9ee3618b55f057297a137)

diff --git a/doc/21-development.md b/doc/21-development.md
index 29e3e3d1b0a5f545053373b28920759e73d48d09..c5ea8e7ccdc6ba81da4dc91870cd493d8005afa7 100644 (file)
--- a/doc/21-development.md
+++ b/doc/21-development.md
@@ -634,7 +634,7 @@ vim /usr/local/icinga2/etc/icinga2/conf.d/api-users.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