Man page fix for unlang

[Alan DeKok]

Chris Moules wrote:
> There is a logic error in the unlang man page.
...
> I was fairly sure that the '=' operator only assigned if the value is
> NOT set. I checked the docs on the wiki
> (http://wiki.freeradius.org/Operators) which states this.
> 
> Below a quick patch.

  Thanks.  Please try to use "git format-patch" if at all possible.  It
helps to avoid mailer issues with spaces, etc.

  I've added the fix.

  Alan DeKok.