Issue 87 in svnbook: ch06: mention 'htdigest' tool when mentioning digest auth

[svnbook at googlecode.com]

Comment #25 on issue 87 by cmpil... at gmail.com: ch06: mention 'htdigest'  
tool when mentioning digest auth
http://code.google.com/p/svnbook/issues/detail?id=87

Actually, I was able to make the htdigest patch apply with some relatively  
painless hand-editing of the patch file.  Just reviewing the final results  
now.