README.txt: revert hardcoded revision number.

git-svn-id: http://framework.zend.com/svn/framework/standard/trunk@15527 44c647ce-9c0f-0410-b52a-842ac1e357ba

README.txt

@@ -2,7 +2,7 @@ Welcome to the Zend Framework 1.8 Release!
 
 RELEASE INFORMATION
 ---------------
-Zend Framework 1.8 Release (revision 15226).
+Zend Framework 1.8 Release ([INSERT REV NUM HERE]).
 Released on April 30, 2009.
 
 NEW FEATURES