Update PostgreSQL createuser example

Use -RDIElPS flags when creating a new user within the database.

This is a more restricted account than what was previously suggested
in the documentation, reducing some potential risks.

Bug: issue 1875
Change-Id: I4eab2ce2ceed2390c50d20c5bef25432c6d2cc41
diff --git a/Documentation/database-setup.txt b/Documentation/database-setup.txt
index 0465d62..c559b0e 100644
--- a/Documentation/database-setup.txt
+++ b/Documentation/database-setup.txt
@@ -33,7 +33,7 @@
 full rights on the newly created database:
 
 ----
-  $ createuser --username=postgres -S -R -D -P -E gerrit2
+  $ createuser --username=postgres -RDIElPS gerrit2
   $ createdb --username=postgres -E UTF-8 -O gerrit2 reviewdb
 ----