Add a missing \define I accidentally assumed was there in r9592.

[originally from svn r9593]
[r9592 == 71f0c7546d]

doc/blurb.but

@@ -1,5 +1,7 @@
 \define{versionidblurb} \versionid $Id$
 
+\define{dash} \u2013{-}
+
 \title PuTTY User Manual
 
 \cfg{xhtml-leaf-level}{1}