doc: Simplify example of HISTFILE for psql

e4c76191 has added a space to the example used for HISTFILE in the docs
of psql before the variable DBNAME, as a workaround because variables
were not parsed the same way back then.  This behavior has changed in
9.2, causing the example in the psql docs to result in the same history
file created with or without a space added before the DBNAME variable.

Let's just remove this space in the example, to reduce any confusion, as
the point of it is to prove that a per-database history file is easy to
set up, and that's easier to read this way.

Per discussion with Tom Lane.

Reported-by: Ludovic Kuty
Discussion: https://postgr.es/m/161830067409.691.16198363670687811485@wrigleys.postgresql.org

doc/src/sgml/ref/psql-ref.sgml

@@ -3915,7 +3915,7 @@ bar
         or <filename>%APPDATA%\postgresql\psql_history</filename> on Windows.
         For example, putting:
 <programlisting>
-\set HISTFILE ~/.psql_history- :DBNAME
+\set HISTFILE ~/.psql_history-:DBNAME
 </programlisting>
         in <filename>~/.psqlrc</filename> will cause
         <application>psql</application> to maintain a separate history for