Tweak a comment

Author: David Robertson

.ci/scripts/test_synapse_port_db.sh

@@ -4,8 +4,9 @@
 #   - sets up synapse and deps
 #   - runs the port script on a prepopulated test sqlite db
 #   - also runs it against an new sqlite db
-
-# Expects Synapse to have been already installed with `poetry install -e .[postgres]
+#
+# Expects Synapse to have been already installed with `poetry install --extras postgres`.
+# Expects `poetry` to be available on the `PATH`.
 
 set -xe
 cd "$(dirname "$0")/../.."