Update docker command for sample poostgresql

As the default postgresql version had been upgraded as version 12,  the sample command should be updating too, otherwise, we will meet permission issue during first execution.

Author: alaahong

README.md

@@ -170,7 +170,7 @@ Step 1. Launch a postgresql container
 docker run --name gitlab-postgresql -d \
     --env 'DB_NAME=gitlabhq_production' \
     --env 'DB_USER=gitlab' --env 'DB_PASS=password' \
-    --env 'DB_EXTENSION=pg_trgm' \
+    --env 'DB_EXTENSION=pg_trgm,btree_gist' \
     --volume /srv/docker/gitlab/postgresql:/var/lib/postgresql \
     sameersbn/postgresql:12-20200524
 ```