README package installation instruction adjustment

Author: gsmolk

README.md

@@ -52,12 +52,13 @@ apt-get install pg-probackup-(10|9.6|9.5)
 #DEB-SRC Packages
 echo "deb-src [arch=amd64] http://repo.postgrespro.ru/pg_probackup/deb/ $(lsb_release -cs) main-$(lsb_release -cs)" >>\
   /etc/apt/sources.list.d/pg_probackup.list
-```
 
-```shell
 #RPM Centos Packages
 rpm -ivh http://repo.postgrespro.ru/pg_probackup/keys/pg_probackup-repo-centos.noarch.rpm
 yum install pg_probackup-(10|9.6|9.5)
+
+#SRPM Centos Packages
+yumdownloader --source pg_probackup-(10|9.6|9.5)
 ```
 
 To compile `pg_probackup`, you must have a PostgreSQL installation and raw source tree. To install `pg_probackup`, execute this in the module's directory: