copy-edits an exception message

Inserted spaces in the name of Rails components.

Since I was on it, also used PostgreSQL instead of Postgres
because albeit Postgres is an accepted alias, PostgreSQL is
the official name and the actual name of the adapter.

See

    https://wiki.postgresql.org/wiki/ProjectName

with regard to PostgreSQL vs Postgres.

actioncable/lib/action_cable/subscription_adapter/postgresql.rb

@@ -33,7 +33,7 @@ def with_connection(&block) # :nodoc:
           pg_conn = ar_conn.raw_connection
 
           unless pg_conn.is_a?(PG::Connection)
-            raise "ActiveRecord database must be Postgres in order to use the Postgres ActionCable storage adapter"
+            raise "The Active Record database must be PostgreSQL in order to use the PostgreSQL Action Cable storage adapter"
           end
 
           yield pg_conn