Fix documentation to create the `pg_trm` extension before creating the DB

[ci skip]
Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/install/installation.md

@@ -176,16 +176,16 @@ We recommend using a PostgreSQL database. For MySQL check the
     sudo -u postgres psql -d template1 -c "CREATE USER git CREATEDB;"
     ```
     
-1. Create the GitLab production database and grant all privileges on database:
+1. Create the `pg_trgm` extension (required for GitLab 8.6+):
 
     ```bash
-    sudo -u postgres psql -d template1 -c "CREATE DATABASE gitlabhq_production OWNER git;"
+    sudo -u postgres psql -d template1 -c "CREATE EXTENSION IF NOT EXISTS pg_trgm;"
     ```
 
-1. Create the `pg_trgm` extension (required for GitLab 8.6+):
+1. Create the GitLab production database and grant all privileges on database:
 
     ```bash
-    sudo -u postgres psql -d template1 -c "CREATE EXTENSION IF NOT EXISTS pg_trgm;"
+    sudo -u postgres psql -d template1 -c "CREATE DATABASE gitlabhq_production OWNER git;"
     ```
 
 1. Try connecting to the new database with the new user: