Fix a minor typo.

contrib/chkpass/README.chkpass

-$PostgreSQL: pgsql/contrib/chkpass/README.chkpass,v 1.3 2005/09/23 15:05:04 tgl Exp $
+$PostgreSQL: pgsql/contrib/chkpass/README.chkpass,v 1.4 2007/10/01 06:52:42 darcy Exp $
 
 Chkpass is a password type that is automatically checked and converted upon
-entry.  It is stored encrypted.  To compare, simply compare agains a clear
+entry.  It is stored encrypted.  To compare, simply compare against a clear
 text password and the comparison function will encrypt it before comparing.
 It also returns an error if the code determines that the password is easily
 crackable.  This is currently a stub that does nothing.