Modify shell command which configures PostgreSQL repo to match that used in PostgreSQL docs

doc/install/installation.md

@@ -287,9 +287,9 @@ In GitLab 12.1 and later, only PostgreSQL is supported. In GitLab 13.0 and later
 
    ```shell
    wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -
-   RELEASE=$(lsb_release -cs) echo "deb http://apt.postgresql.org/pub/repos/apt/ ${RELEASE}"-pgdg main | sudo tee  /etc/apt/sources.list.d/pgdg.list
+   sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt $(lsb_release -cs)-pgdg main" > /etc/apt/sources.list.d/pgdg.list'
    sudo apt update
-   sudo apt -y install postgresql-11 postgresql-client-11 libpq-dev
+   sudo apt -y install postgresql-12 postgresql-client-12 libpq-dev
    ```
 
 1. Verify the PostgreSQL version you have is supported by the version of GitLab you're