Readme: formatting fixes

Author: gsmolk

README.rst

@@ -20,11 +20,11 @@ Installation
 
 DEB packages for Debian|Ubuntu:
 
-```
+.. code-block:: bash
     echo "deb [arch=amd64] https://repo.postgrespro.ru/mamonsu/latest/deb/ $(lsb_release -cs) main-$(lsb_release -cs)" > /etc/apt/sources.list.d/mamonsu.list
     wget -O - https://repo.postgrespro.ru/mamonsu/keys/GPG-KEY-MAMONSU | sudo apt-key add - && sudo apt-get update
     sudo apt-get install mamonsu
-```
+
 
 RPM packages for Centos: